Merge branch 'css' of github.com:Textualize/textual into text-input-cursor-to-click

2025-10-17 02:38:12 +03:00 · 2022-05-25 14:01:11 +01:00
parent 858e718dde 2b485cd4cc
commit b0e197f446
41 changed files with 826 additions and 403 deletions
--- a/docs/examples/actions/colorizer.py
+++ b/docs/examples/actions/colorizer.py
@@ -1,14 +0,0 @@
-from textual.app import App
-
-
-class Colorizer(App):
-    async def on_load(self):
-        await self.bind("r", "color('red')")
-        await self.bind("g", "color('green')")
-        await self.bind("b", "color('blue')")
-
-    def action_color(self, color: str) -> None:
-        self.background = f"on {color}"
-
-
-Colorizer.run()
--- a/docs/examples/actions/quiter.py
+++ b/docs/examples/actions/quiter.py
@@ -1,9 +0,0 @@
-from textual.app import App
-
-
-class Quiter(App):
-    async def on_load(self):
-        await self.bind("q", "quit")
-
-
-Quiter.run()
--- a/docs/examples/messages_and_events/beep.py
+++ b/docs/examples/messages_and_events/beep.py
@@ -1,9 +0,0 @@
-from textual.app import App
-
-
-class Beeper(App):
-    def on_key(self):
-        self.console.bell()
-
-
-Beeper.run()
--- a/docs/examples/messages_and_events/beep_typed.py
+++ b/docs/examples/messages_and_events/beep_typed.py
@@ -1,10 +0,0 @@
-from textual.app import App
-from textual import events
-
-
-class Beeper(App):
-    async def on_key(self, event: events.Key) -> None:
-        self.console.bell()
-
-
-Beeper.run()
--- a/docs/examples/messages_and_events/color_changer.py
+++ b/docs/examples/messages_and_events/color_changer.py
@@ -1,10 +0,0 @@
-from textual.app import App
-
-
-class ColorChanger(App):
-    def on_key(self, event):
-        if event.key.isdigit():
-            self.background = f"on color({event.key})"
-
-
-ColorChanger.run(log_path="textual.log")
--- a/docs/examples/simple.css
+++ b/docs/examples/simple.css
@@ -0,0 +1,14 @@
+Screen {
+    background: darkblue;
+    color: white;
+    layout: vertical;
+    align: center middle;
+}
+Static {
+    height: auto;
+    padding: 2;
+    margin: 2;
+    border:  white;
+    background: #ffffff 30%;
+    content-align: center middle;    
+}
--- a/docs/examples/simple.py
+++ b/docs/examples/simple.py
@@ -0,0 +1,11 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class TextApp(App):
+    def compose(self) -> ComposeResult:
+        yield Static("Hello")
+        yield Static("[b]World![/b]")
+
+
+app = TextApp(css_path="simple.css")
--- a/docs/examples/timers/clock.py
+++ b/docs/examples/timers/clock.py
@@ -1,24 +0,0 @@
-from datetime import datetime
-
-from rich.align import Align
-from rich.style import Style
-
-from textual.app import App
-from textual.widget import Widget
-
-
-class Clock(Widget):
-    def on_mount(self):
-        self.set_interval(1, self.refresh)
-
-    def render(self, style: Style):
-        time = datetime.now().strftime("%c")
-        return Align.center(time, vertical="middle")
-
-
-class ClockApp(App):
-    async def on_mount(self):
-        await self.screen.dock(Clock())
-
-
-ClockApp.run()
--- a/docs/examples/widgets/custom.py
+++ b/docs/examples/widgets/custom.py
@@ -1,33 +0,0 @@
-from rich.panel import Panel
-from rich.style import Style
-
-from textual.app import App
-from textual.reactive import Reactive
-from textual.widget import Widget
-
-
-class Hover(Widget):
-
-    mouse_over = Reactive(False)
-
-    def render(self, style: Style) -> Panel:
-        return Panel("Hello [b]World[/b]", style=("on red" if self.mouse_over else ""))
-
-    def on_enter(self) -> None:
-        self.mouse_over = True
-
-    def on_leave(self) -> None:
-        self.mouse_over = False
-
-
-class HoverApp(App):
-    """Demonstrates smooth animation"""
-
-    async def on_mount(self) -> None:
-        """Build layout here."""
-
-        hovers = (Hover() for _ in range(10))
-        await self.screen.dock(*hovers, edge="top")
-
-
-HoverApp.run(log_path="textual.log")
--- a/docs/examples/widgets/placeholders.py
+++ b/docs/examples/widgets/placeholders.py
@@ -1,15 +0,0 @@
-from textual.app import App
-from textual.widgets import Placeholder
-
-
-class SimpleApp(App):
-    """Demonstrates smooth animation"""
-
-    async def on_mount(self) -> None:
-        """Build layout here."""
-
-        await self.screen.dock(Placeholder(), edge="left", size=40)
-        await self.screen.dock(Placeholder(), Placeholder(), edge="top")
-
-
-SimpleApp.run(log_path="textual.log")
--- a/docs/index.md
+++ b/docs/index.md
@@ -4,8 +4,27 @@ Textual is framework for rapidly creating _text user interfaces_ (TUIs from here

 A TUI is an application that lives within a terminal, which can have mouse and keyboard support and user interface elements like windows and panels, but is rendered purely with text. They have a number of advantages over GUI applications: they can be launched from the command line, and return to the command line, and they work over ssh.

+## Foo
+
 Creating a TUI can be challenging. It may be easier to create a GUI or web application than it is to build a TUI with traditional techniques. Often projects that could use one or the other never manage to ship either.

-Textual seeks to lower the difficulty level of building a TUI by borrowing developments from the web world and to a lesser extent desktop applications. The goal is for it to be as easy to develop a TUI for your project as it would be to add a command line interface.
+Textual seeks to lower the difficulty level of building a TUI by borrowing developments from the web world and to a lesser extent desktop applications. The goal is for it to be as easy to develop a TUI for your project as it would be to add a command line interface.XX
+
+=== "simple.py"
+
+    ```python
+    --8<-- "docs/examples/simple.py"
+    ```
+
+=== "simple.css"
+
+    ```scss
+    --8<-- "docs/examples/simple.css"
+    ```
+
+=== "Result"
+
+    ```{.textual path="docs/examples/simple.py" columns="80" lines="24"}
+    ```

 Textual also offers a number of enhancements over traditional TUI applications by taking advantage of improvements to terminal software and the hardware it runs on. Terminals are a far cry from their roots in ancient hardware and dial-up modems, yet much of the software that runs on them hasn't kept pace.
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -1,12 +1,26 @@
 site_name: Textual
-site_url: https://example.com/
+site_url: https://www.textualize.io/

 markdown_extensions:
-  - pymdownx.highlight
-  - pymdownx.superfences
+  - admonition
+  - meta
+  - toc:
+      permalink: true
+  - pymdownx.keys
+  - pymdownx.highlight:
+      anchor_linenums: true
+  - pymdownx.superfences:
+      custom_fences:
+        - name: textual
+          class: textual
+          format: !!python/name:textual._doc.format_svg
  - pymdownx.inlinehilite
  - pymdownx.superfences
  - pymdownx.snippets
+  - pymdownx.tabbed:
+      alternate_style: true 
+  - pymdownx.snippets
+  - markdown.extensions.attr_list

 theme:
  name: material
--- a/poetry.lock
+++ b/poetry.lock
@@ -166,7 +166,7 @@ test = ["flake8 (==3.7.8)", "hypothesis (==3.55.3)"]

 [[package]]
 name = "coverage"
-version = "6.3.3"
+version = "6.4"
 description = "Code coverage measurement for Python"
 category = "dev"
 optional = false
@@ -219,7 +219,7 @@ dev = ["twine", "markdown", "flake8", "wheel"]

 [[package]]
 name = "identify"
-version = "2.5.0"
+version = "2.5.1"
 description = "File identification library for Python"
 category = "dev"
 optional = false
@@ -238,7 +238,7 @@ python-versions = ">=3.5"

 [[package]]
 name = "importlib-metadata"
-version = "4.11.3"
+version = "4.11.4"
 description = "Read metadata from Python packages"
 category = "main"
 optional = false
@@ -263,11 +263,11 @@ python-versions = "*"

 [[package]]
 name = "jinja2"
-version = "3.1.2"
+version = "3.0.3"
 description = "A very fast and expressive template engine."
 category = "dev"
 optional = false
-python-versions = ">=3.7"
+python-versions = ">=3.6"

 [package.dependencies]
 MarkupSafe = ">=2.0"
@@ -650,7 +650,7 @@ pyyaml = "*"

 [[package]]
 name = "rich"
-version = "12.4.1"
+version = "12.4.4"
 description = "Render rich text, tables, progress bars, syntax highlighting, markdown and more to the terminal"
 category = "main"
 optional = false
@@ -701,7 +701,7 @@ python-versions = ">=3.7"

 [[package]]
 name = "typed-ast"
-version = "1.5.3"
+version = "1.5.4"
 description = "a fork of Python 2 and 3 ast modules with type comment support"
 category = "dev"
 optional = false
@@ -773,7 +773,7 @@ testing = ["pytest (>=6)", "pytest-checkdocs (>=2.4)", "pytest-flake8", "pytest-
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.7"
-content-hash = "3579be8d55deb729ef79984823765900552e19284c205b7bedc92897190fb6fd"
+content-hash = "eba121f02e102fd9c551a654bcfab3028ec4fc05fe9b4cf7d5f64002e3586ba0"

 [metadata.files]
 aiohttp = [
@@ -924,47 +924,47 @@ commonmark = [
    {file = "commonmark-0.9.1.tar.gz", hash = "sha256:452f9dc859be7f06631ddcb328b6919c67984aca654e5fefb3914d54691aed60"},
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+    {file = "typed_ast-1.5.4-cp38-cp38-macosx_11_0_arm64.whl", hash = "sha256:7d5d014b7daa8b0bf2eaef684295acae12b036d79f54178b92a2b6a56f92278f"},
+    {file = "typed_ast-1.5.4-cp38-cp38-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:370788a63915e82fd6f212865a596a0fefcbb7d408bbbb13dea723d971ed8bdc"},
+    {file = "typed_ast-1.5.4-cp38-cp38-manylinux_2_5_x86_64.manylinux1_x86_64.manylinux_2_12_x86_64.manylinux2010_x86_64.whl", hash = "sha256:4e964b4ff86550a7a7d56345c7864b18f403f5bd7380edf44a3c1fb4ee7ac6c6"},
+    {file = "typed_ast-1.5.4-cp38-cp38-win_amd64.whl", hash = "sha256:683407d92dc953c8a7347119596f0b0e6c55eb98ebebd9b23437501b28dcbb8e"},
+    {file = "typed_ast-1.5.4-cp39-cp39-macosx_10_9_x86_64.whl", hash = "sha256:4879da6c9b73443f97e731b617184a596ac1235fe91f98d279a7af36c796da35"},
+    {file = "typed_ast-1.5.4-cp39-cp39-macosx_11_0_arm64.whl", hash = "sha256:3e123d878ba170397916557d31c8f589951e353cc95fb7f24f6bb69adc1a8a97"},
+    {file = "typed_ast-1.5.4-cp39-cp39-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:ebd9d7f80ccf7a82ac5f88c521115cc55d84e35bf8b446fcd7836eb6b98929a3"},
+    {file = "typed_ast-1.5.4-cp39-cp39-manylinux_2_5_x86_64.manylinux1_x86_64.manylinux_2_12_x86_64.manylinux2010_x86_64.whl", hash = "sha256:98f80dee3c03455e92796b58b98ff6ca0b2a6f652120c263efdba4d6c5e58f72"},
+    {file = "typed_ast-1.5.4-cp39-cp39-win_amd64.whl", hash = "sha256:0fdbcf2fef0ca421a3f5912555804296f0b0960f0418c440f5d6d3abb549f3e1"},
+    {file = "typed_ast-1.5.4.tar.gz", hash = "sha256:39e21ceb7388e4bb37f4c679d72707ed46c2fbf2a5609b8b8ebc4b067d977df2"},
 ]
 typing-extensions = [
    {file = "typing_extensions-4.2.0-py3-none-any.whl", hash = "sha256:6657594ee297170d19f67d55c05852a874e7eb634f4f753dbd667855e07c1708"},
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -22,9 +22,8 @@ textual = "textual.cli.cli:run"

 [tool.poetry.dependencies]
 python = "^3.7"
-rich = "^12.4.0"
-
-#rich = {git = "git@github.com:willmcgugan/rich", rev = "link-id"}
+rich = "^12.4.3"
+#rich = {path="../rich", develop=true}
 click = "8.1.2"
 importlib-metadata = "^4.11.3"
 typing-extensions = { version = "^4.0.0", python = "<3.8" }
@@ -35,12 +34,13 @@ pytest = "^6.2.3"
 black = "^22.3.0"
 mypy = "^0.950"
 pytest-cov = "^2.12.1"
-mkdocs = "^1.2.3"
+mkdocs = "^1.3.0"
 mkdocstrings = "^0.17.0"
 mkdocs-material = "^7.3.6"
 pre-commit = "^2.13.0"
 pytest-aiohttp = "^1.0.4"
 time-machine = "^2.6.0"
+Jinja2 = "<3.1.0"

 [tool.black]
 includes = "src"
@@ -49,7 +49,7 @@ includes = "src"
 asyncio_mode = "auto"
 testpaths = ["tests"]
 markers = [
-    "integration_test: marks tests as slow integration tests(deselect with '-m \"not integration_test\"')",
+    "integration_test: marks tests as slow integration tests (deselect with '-m \"not integration_test\"')",
 ]

 [build-system]
--- a/sandbox/basic.css
+++ b/sandbox/basic.css
@@ -84,7 +84,7 @@ Tweet {
  /* border: outer $primary; */
  padding: 1;
  border: wide $panel-darken-2;
-  overflow-y: auto;
+  overflow: auto;
  /* scrollbar-gutter: stable; */
  align-horizontal: center;
  box-sizing: border-box;
@@ -114,10 +114,10 @@ TweetHeader {
 }

 TweetBody {  
-  width: 100w;
+  width: 130%;
  background: $panel;
  color: $text-panel;
-  height: auto;
+  height: auto; 
  padding: 0 1 0 0; 
 
 }
--- a/sandbox/basic.py
+++ b/sandbox/basic.py
@@ -91,7 +91,7 @@ class Success(Widget):
        return Text("This is a success message", justify="center")


-class BasicApp(App):
+class BasicApp(App, css_path="basic.css"):
    """A basic app demonstrating CSS"""

    def on_load(self):
@@ -158,11 +158,7 @@ class BasicApp(App):
        tweet_body.refresh(layout=True)


-app = BasicApp(
-    css_path="basic.css",
-    watch_css=True,
-    log_path="textual.log",
-)
+app = BasicApp()

 if __name__ == "__main__":
    app.run()
--- a/sandbox/nest.py
+++ b/sandbox/nest.py
@@ -16,7 +16,7 @@ class TextWidget(Widget):
        return TEXT


-class AutoApp(App):
+class AutoApp(App, css_path="nest.css"):
    def on_mount(self) -> None:
        self.bind("t", "tree")

@@ -30,9 +30,3 @@ class AutoApp(App):

    def action_tree(self):
        self.log(self.screen.tree)
-
-
-app = AutoApp(css_path="nest.css")
-
-if __name__ == "__main__":
-    app.run()
--- a/sandbox/scroll_to_widget.py
+++ b/sandbox/scroll_to_widget.py
@@ -35,7 +35,7 @@ class Introduction(Widget):
    }
    """

-    def render(self) -> RenderableType:
+    def render(self, styles) -> RenderableType:
        return Text(
            "Press keys 0 to 9 to scroll to the Placeholder with that ID.",
            justify="center",
--- a/sandbox/simplest.py
+++ b/sandbox/simplest.py
@@ -0,0 +1,3 @@
+from textual.app import App
+
+app = App()
--- a/src/textual/_animator.py
+++ b/src/textual/_animator.py
@@ -3,14 +3,12 @@ from __future__ import annotations
 from abc import ABC, abstractmethod
 import asyncio
 import sys
-from time import monotonic
 from typing import Any, Callable, TypeVar

 from dataclasses import dataclass

-from . import log
+from . import _clock
 from ._easing import DEFAULT_EASING, EASING
-from ._profile import timer
 from ._timer import Timer
 from ._types import MessageTarget

@@ -139,10 +137,6 @@ class Animator:
            pause=True,
        )

-    def get_time(self) -> float:
-        """Get the current wall clock time."""
-        return monotonic()
-
    async def start(self) -> None:
        """Start the animator task."""

@@ -186,10 +180,13 @@ class Animator:
            raise AttributeError(
                f"Can't animate attribute {attribute!r} on {obj!r}; attribute does not exist"
            )
+        assert (duration is not None and speed is None) or (
+            duration is None and speed is not None
+        ), "An Animation should have a duration OR a speed"

        if final_value is ...:
            final_value = value
-        start_time = self.get_time()
+        start_time = self._get_time()

        animation_key = (id(obj), attribute)

@@ -240,9 +237,15 @@ class Animator:
        if not self._animations:
            self._timer.pause()
        else:
-            animation_time = self.get_time()
+            animation_time = self._get_time()
            animation_keys = list(self._animations.keys())
            for animation_key in animation_keys:
                animation = self._animations[animation_key]
                if animation(animation_time):
                    del self._animations[animation_key]
+
+    def _get_time(self) -> float:
+        """Get the current wall clock time, via the internal Timer."""
+        # N.B. We could remove this method and always call `self._timer.get_time()` internally,
+        # but it's handy to have in mocking situations
+        return _clock.get_time_no_wait()
--- a/src/textual/_ansi_sequences.py
+++ b/src/textual/_ansi_sequences.py
@@ -47,7 +47,7 @@ ANSI_SEQUENCES: Dict[str, Tuple[Keys, ...]] = {
    # support it. (Most terminals send ControlH when backspace is pressed.)
    # See: http://www.ibb.net/~anne/keyboard.html
    "\x7f": (Keys.ControlH,),
-    # --
+    "\x1b\x7f": (Keys.ControlW,),
    # Various
    "\x1b[1~": (Keys.Home,),  # tmux
    "\x1b[2~": (Keys.Insert,),
--- a/src/textual/_clock.py
+++ b/src/textual/_clock.py
@@ -0,0 +1,58 @@
+import asyncio
+from time import monotonic
+
+
+"""
+A module that serves as the single source of truth for everything time-related in a Textual app.
+Having this logic centralised makes it easier to simulate time in integration tests, 
+by mocking the few functions exposed by this module.
+"""
+
+
+# N.B. This class and its singleton instance have to be hidden APIs because we want to be able to mock time,
+# even for Python modules that imported functions such as `get_time` *before* we mocked this internal _Clock.
+# (so mocking public APIs such as `get_time` wouldn't affect direct references to then that were done during imports)
+class _Clock:
+    async def get_time(self) -> float:
+        return self.get_time_no_wait()
+
+    def get_time_no_wait(self) -> float:
+        return monotonic()
+
+    async def sleep(self, seconds: float) -> None:
+        await asyncio.sleep(seconds)
+
+
+# That's our target for mocking time! :-)
+_clock = _Clock()
+
+
+def get_time_no_wait() -> float:
+    """
+    Get the current wall clock time.
+
+    Returns:
+        float: the value (in fractional seconds) of a monotonic clock, i.e. a clock that cannot go backwards.
+    """
+    return _clock.get_time_no_wait()
+
+
+async def get_time() -> float:
+    """
+    Asynchronous version of `get_time`. Useful in situations where we want asyncio to be
+    able to "do things" elsewhere right before we fetch the time.
+
+    Returns:
+        float: the value (in fractional seconds) of a monotonic clock, i.e. a clock that cannot go backwards.
+    """
+    return await _clock.get_time()
+
+
+async def sleep(seconds: float) -> None:
+    """
+    Coroutine that completes after a given time (in seconds).
+
+    Args:
+        seconds (float): the duration we should wait for before unblocking the awaiter
+    """
+    return await _clock.sleep(seconds)
--- a/src/textual/_compositor.py
+++ b/src/textual/_compositor.py
@@ -552,7 +552,7 @@ class Compositor:
        ]
        return segment_lines

-    def render(self) -> RenderableType:
+    def render(self, full: bool = True) -> RenderableType:
        """Render a layout.

        Returns:
@@ -561,11 +561,14 @@ class Compositor:
        width, height = self.size
        screen_region = Region(0, 0, width, height)

-        update_regions = self._dirty_regions.copy()
-        self._dirty_regions.clear()
-        if screen_region in update_regions:
-            # If one of the updates is the entire screen, then we only need one update
-            update_regions.clear()
+        if full:
+            update_regions: set[Region] = set()
+        else:
+            update_regions = self._dirty_regions.copy()
+            self._dirty_regions.clear()
+            if screen_region in update_regions:
+                # If one of the updates is the entire screen, then we only need one update
+                update_regions.clear()

        if update_regions:
            # Create a crop regions that surrounds all updates
--- a/src/textual/_context.py
+++ b/src/textual/_context.py
@@ -5,4 +5,9 @@ from contextvars import ContextVar
 if TYPE_CHECKING:
    from .app import App

+
+class NoActiveAppError(RuntimeError):
+    pass
+
+
 active_app: ContextVar["App"] = ContextVar("active_app")
--- a/src/textual/_doc.py
+++ b/src/textual/_doc.py
@@ -0,0 +1,36 @@
+import os
+
+
+# This module defines our "Custom Fences", powered by SuperFences
+# @link https://facelessuser.github.io/pymdown-extensions/extensions/superfences/#custom-fences
+def format_svg(source, language, css_class, options, md, attrs, **kwargs):
+    """A superfences formatter to insert a SVG screenshot."""
+
+    os.environ["TEXTUAL"] = "headless"
+    os.environ["TEXTUAL_SCREENSHOT"] = "0.1"
+    os.environ["COLUMNS"] = attrs.get("columns", "80")
+    os.environ["LINES"] = attrs.get("lines", "24")
+    path = attrs.get("path")
+
+    if path:
+        cwd = os.getcwd()
+        examples_path, filename = os.path.split(path)
+        try:
+            os.chdir(examples_path)
+            with open(filename, "rt") as python_code:
+                source = python_code.read()
+            app_vars = {}
+            exec(source, app_vars)
+            app = app_vars["app"]
+            app.run()
+            svg = app._screenshot
+
+        finally:
+            os.chdir(cwd)
+    else:
+        app_vars = {}
+        exec(source, app_vars)
+        app = app_vars["app"]
+        app.run()
+        svg = app._screenshot
+    return svg
--- a/src/textual/_timer.py
+++ b/src/textual/_timer.py
@@ -5,17 +5,15 @@ import weakref
 from asyncio import (
    CancelledError,
    Event,
-    sleep,
    Task,
 )
-from functools import partial
-from time import monotonic
 from typing import Awaitable, Callable, Union

 from rich.repr import Result, rich_repr

 from . import events
 from ._callback import invoke
+from . import _clock
 from ._types import MessageTarget

 TimerCallback = Union[Callable[[], Awaitable[None]], Callable[[], None]]
@@ -109,32 +107,38 @@ class Timer:
        count = 0
        _repeat = self._repeat
        _interval = self._interval
-        start = monotonic()
+        start = _clock.get_time_no_wait()
        try:
            while _repeat is None or count <= _repeat:
                next_timer = start + ((count + 1) * _interval)
-                if self._skip and next_timer < monotonic():
+                now = await _clock.get_time()
+                if self._skip and next_timer < now:
                    count += 1
                    continue
-                wait_time = max(0, next_timer - monotonic())
+                now = await _clock.get_time()
+                wait_time = max(0, next_timer - now)
                if wait_time:
-                    await sleep(wait_time)
-                event = events.Timer(
-                    self.sender,
-                    timer=self,
-                    time=next_timer,
-                    count=count,
-                    callback=self._callback,
-                )
+                    await _clock.sleep(wait_time)
                count += 1
                try:
-                    if self._callback is not None:
-                        await invoke(self._callback)
-                    else:
-                        await self.target.post_priority_message(event)
-
+                    await self._tick(next_timer=next_timer, count=count)
                except EventTargetGone:
                    break
                await self._active.wait()
        except CancelledError:
            pass
+
+    async def _tick(self, *, next_timer: float, count: int) -> None:
+        """Triggers the Timer's action: either call its callback, or sends an event to its target"""
+        if self._callback is not None:
+            await invoke(self._callback)
+        else:
+            event = events.Timer(
+                self.sender,
+                timer=self,
+                time=next_timer,
+                count=count,
+                callback=self._callback,
+            )
+
+            await self.target.post_priority_message(event)
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -109,6 +109,8 @@ class App(Generic[ReturnType], DOMNode):

    """

+    CSS_PATH: str | None = None
+
    def __init__(
        self,
        driver_class: Type[Driver] | None = None,
@@ -136,8 +138,13 @@ class App(Generic[ReturnType], DOMNode):
        # this will create some first references to an asyncio loop.
        _init_uvloop()

+        self.features: frozenset[FeatureFlag] = parse_features(os.getenv("TEXTUAL", ""))
+
        self.console = Console(
-            file=sys.__stdout__, markup=False, highlight=False, emoji=False
+            file=(open(os.devnull, "wt") if self.is_headless else sys.__stdout__),
+            markup=True,
+            highlight=False,
+            emoji=False,
        )
        self.error_console = Console(markup=False, stderr=True)
        self.driver_class = driver_class or self.get_driver_class()
@@ -183,10 +190,7 @@ class App(Generic[ReturnType], DOMNode):

        self.stylesheet = Stylesheet(variables=self.get_css_variables())
        self._require_styles_update = False
-
-        self.css_path = css_path
-
-        self.features: frozenset[FeatureFlag] = parse_features(os.getenv("TEXTUAL", ""))
+        self.css_path = css_path or self.CSS_PATH

        self.registry: set[MessagePump] = set()
        self.devtools = DevtoolsClient()
@@ -200,6 +204,10 @@ class App(Generic[ReturnType], DOMNode):

        super().__init__()

+    def __init_subclass__(cls, css_path: str | None = None) -> None:
+        super().__init_subclass__()
+        cls.CSS_PATH = css_path
+
    title: Reactive[str] = Reactive("Textual")
    sub_title: Reactive[str] = Reactive("")
    background: Reactive[str] = Reactive("black")
@@ -215,6 +223,11 @@ class App(Generic[ReturnType], DOMNode):
        """Check if debug mode is enabled."""
        return "debug" in self.features

+    @property
+    def is_headless(self) -> bool:
+        """Check if the app is running in 'headless' mode."""
+        return "headless" in self.features
+
    def exit(self, result: ReturnType | None = None) -> None:
        """Exit the app, and return the supplied result.

@@ -430,7 +443,7 @@ class App(Generic[ReturnType], DOMNode):
            color_system="truecolor",
            record=True,
        )
-        console.print(self.screen._compositor)
+        console.print(self.screen._compositor.render(full=True))
        return console.export_svg(title=self.title)

    def save_screenshot(self, path: str | None = None) -> str:
@@ -443,6 +456,7 @@ class App(Generic[ReturnType], DOMNode):
        Returns:
            str: Filename of screenshot.
        """
+        self.bell()
        if path is None:
            svg_path = f"{self.title.lower()}_{datetime.now().isoformat()}.svg"
            svg_path = svg_path.replace("/", "_").replace("\\", "_")
@@ -727,13 +741,12 @@ class App(Generic[ReturnType], DOMNode):
                mount_event = events.Mount(sender=self)
                await self.dispatch_message(mount_event)

-                # TODO: don't override `self.console` here
-                self.console = Console(file=sys.__stdout__)
                self.title = self._title
                self.refresh()
                await self.animator.start()

                with redirect_stdout(StdoutRedirector(self.devtools, self._log_file)):  # type: ignore
+                    await self._ready()
                    await super().process_messages()
                    await self.animator.stop()
                    await self.close_all()
@@ -754,6 +767,28 @@ class App(Generic[ReturnType], DOMNode):
                self._log_file.close()
                self._log_console = None

+    async def _ready(self) -> None:
+        """Called immediately prior to processing messages.
+
+        May be used as a hook for any operations that should run first.
+
+        """
+        try:
+            screenshot_timer = float(os.environ.get("TEXTUAL_SCREENSHOT", "0"))
+        except ValueError:
+            return
+
+        if not screenshot_timer:
+            return
+
+        async def on_screenshot():
+            """Used by docs plugin."""
+            svg = self.export_screenshot()
+            self._screenshot = svg  # type: ignore
+            await self.shutdown()
+
+        self.set_timer(screenshot_timer, on_screenshot)
+
    def on_mount(self) -> None:
        widgets = list(self.compose())
        if widgets:
@@ -856,7 +891,7 @@ class App(Generic[ReturnType], DOMNode):
        Args:
            renderable (RenderableType): A Rich renderable.
        """
-        if self._running and not self._closed:
+        if self._running and not self._closed and not self.is_headless:
            console = self.console
            if self._sync_available:
                console.file.write("\x1bP=1s\x1b\\")
--- a/src/textual/box_model.py
+++ b/src/textual/box_model.py
@@ -96,8 +96,8 @@ def get_box_model(
    content_height = max(1, content_height)

    # Get box dimensions by adding gutter
-    width = content_width + gutter.width
-    height = content_height + gutter.height

-    model = BoxModel(Size(width, height), margin)
+    size = Size(content_width, content_height) + gutter.totals
+
+    model = BoxModel(size, margin)
    return model
--- a/src/textual/cli/cli.py
+++ b/src/textual/cli/cli.py
@@ -1,8 +1,16 @@
-import click
+from __future__ import annotations
+
+from typing import cast, TYPE_CHECKING
+
 from importlib_metadata import version

+import click
+
 from textual.devtools.server import _run_devtools

+if TYPE_CHECKING:
+    from textual.app import App
+

@click.group()
@click.version_option(version("textual"))
@@ -13,3 +21,139 @@ def run():
@run.command(help="Run the Textual Devtools console")
 def console():
    _run_devtools()
+
+
+class AppFail(Exception):
+    pass
+
+
+def import_app(import_name: str) -> App:
+    """Import an app from it's import name.
+
+    Args:
+        import_name (str): A name to import, such as `foo.bar`
+
+    Raises:
+        AppFail: If the app could not be found for any reason.
+
+    Returns:
+        App: A Textual application
+    """
+
+    import inspect
+    import importlib
+    import sys
+
+    from textual.app import App
+
+    lib, _colon, name = import_name.partition(":")
+
+    if lib.endswith(".py"):
+        # We're assuming the user wants to load a .py file
+        try:
+            with open(lib) as python_file:
+                py_code = python_file.read()
+        except Exception as error:
+            raise AppFail(str(error))
+
+        global_vars: dict[str, object] = {}
+        exec(py_code, global_vars)
+
+        if name:
+            # User has given a name, use that
+            try:
+                app = global_vars[name]
+            except KeyError:
+                raise AppFail(f"App {name!r} not found in {lib!r}")
+        else:
+            # User has not given a name
+            if "app" in global_vars:
+                # App exists, lets use that
+                try:
+                    app = global_vars["app"]
+                except KeyError:
+                    raise AppFail(f"App {name!r} not found in {lib!r}")
+            else:
+                # Find a App class or instance that is *not* the base class
+                apps = [
+                    value
+                    for key, value in global_vars.items()
+                    if (
+                        isinstance(value, App)
+                        or (inspect.isclass(value) and issubclass(value, App))
+                        and value is not App
+                    )
+                ]
+                if not apps:
+                    raise AppFail(
+                        f'Unable to find app in {lib!r}, try specifying app with "foo.py:app"'
+                    )
+                if len(apps) > 1:
+                    raise AppFail(
+                        f'Multiple apps found {lib!r}, try specifying app with "foo.py:app"'
+                    )
+                app = apps[0]
+
+    else:
+        # Assuming the user wants to import the file
+        sys.path.append("")
+        try:
+            module = importlib.import_module(lib)
+        except ImportError as error:
+            raise AppFail(str(error))
+
+        try:
+            app = getattr(module, name or "app")
+        except AttributeError:
+            raise AppFail(f"Unable to find {name!r} in {module!r}")
+
+    if inspect.isclass(app) and issubclass(app, App):
+        app = app()
+
+    return cast(App, app)
+
+
+@run.command("run")
+@click.argument("import_name", metavar="FILE or FILE:APP")
+@click.option("--dev", "dev", help="Enable development mode", is_flag=True)
+def run_app(import_name: str, dev: bool) -> None:
+    """Run a Textual app.
+
+    The code to run may be given as a path (ending with .py) or as a Python
+    import, which will load the code and run an app called "app". You may optionally
+    add a colon plus the class or class instance you want to run.
+
+    Here are some examples:
+
+    textual run foo.py
+
+    textual run foo.py:MyApp
+
+    textual run module.foo
+
+    textual run module.foo:MyApp
+
+    """
+
+    import os
+    import sys
+
+    from textual.features import parse_features
+
+    features = set(parse_features(os.environ.get("TEXTUAL", "")))
+    if dev:
+        features.add("debug")
+        features.add("devtools")
+
+    os.environ["TEXTUAL"] = ",".join(sorted(features))
+
+    try:
+        app = import_app(import_name)
+    except AppFail as error:
+        from rich.console import Console
+
+        console = Console(stderr=True)
+        console.print(str(error))
+        sys.exit(1)
+
+    app.run()
--- a/src/textual/dom.py
+++ b/src/textual/dom.py
@@ -9,6 +9,7 @@ from rich.style import Style
 from rich.text import Text
 from rich.tree import Tree

+from ._context import NoActiveAppError
 from ._node_list import NodeList
 from .color import Color
 from .css._error_tools import friendly_list
@@ -463,7 +464,7 @@ class DOMNode(MessagePump):
        try:
            self.app.stylesheet.update(self.app, animate=True)
            self.refresh()
-        except LookupError:
+        except NoActiveAppError:
            pass

    def remove_class(self, *class_names: str) -> None:
@@ -477,7 +478,7 @@ class DOMNode(MessagePump):
        try:
            self.app.stylesheet.update(self.app, animate=True)
            self.refresh()
-        except LookupError:
+        except NoActiveAppError:
            pass

    def toggle_class(self, *class_names: str) -> None:
@@ -491,7 +492,7 @@ class DOMNode(MessagePump):
        try:
            self.app.stylesheet.update(self.app, animate=True)
            self.refresh()
-        except LookupError:
+        except NoActiveAppError:
            pass

    def has_pseudo_class(self, *class_names: str) -> bool:
--- a/src/textual/drivers/linux_driver.py
+++ b/src/textual/drivers/linux_driver.py
@@ -14,8 +14,9 @@ from threading import Event, Thread
 if TYPE_CHECKING:
    from rich.console import Console

-from .. import log
+import rich.repr

+from .. import log
 from .. import events
 from ..driver import Driver
 from ..geometry import Size
@@ -24,6 +25,7 @@ from .._xterm_parser import XTermParser
 from .._profile import timer


+@rich.repr.auto
 class LinuxDriver(Driver):
    """Powers display and input for Linux / MacOS"""

@@ -36,14 +38,19 @@ class LinuxDriver(Driver):
        self.exit_event = Event()
        self._key_thread: Thread | None = None

+    def __rich_repr__(self) -> rich.repr.Result:
+        yield "debug", self._debug
+
    def _get_terminal_size(self) -> tuple[int, int]:
        width: int | None = 80
        height: int | None = 25
+        import shutil
+
        try:
-            width, height = os.get_terminal_size(sys.__stdin__.fileno())
+            width, height = shutil.get_terminal_size()
        except (AttributeError, ValueError, OSError):
            try:
-                width, height = os.get_terminal_size(sys.__stdout__.fileno())
+                width, height = shutil.get_terminal_size()
            except (AttributeError, ValueError, OSError):
                pass
        width = width or 80
--- a/src/textual/features.py
+++ b/src/textual/features.py
@@ -9,16 +9,16 @@ if sys.version_info >= (3, 8):
 else:
    from typing_extensions import Final, Literal

-FEATURES: Final = {"devtools", "debug"}
+FEATURES: Final = {"devtools", "debug", "headless"}

-FeatureFlag = Literal["devtools", "debug"]
+FeatureFlag = Literal["devtools", "debug", "headless"]


 def parse_features(features: str) -> frozenset[FeatureFlag]:
    """Parse features env var

    Args:
-        features (str): Comma seprated feature flags
+        features (str): Comma separated feature flags

    Returns:
        frozenset[FeatureFlag]: A frozen set of known features.
--- a/src/textual/layouts/vertical.py
+++ b/src/textual/layouts/vertical.py
@@ -38,7 +38,12 @@ class VerticalLayout(Layout):
        displayed_children = cast("list[Widget]", parent.displayed_children)
        for widget, box_model, margin in zip(displayed_children, box_models, margins):
            content_width, content_height = box_model.size
-            offset_x = widget.styles.align_width(content_width, size.width)
+            offset_x = (
+                widget.styles.align_width(
+                    content_width, size.width - box_model.margin.width
+                )
+                + box_model.margin.left
+            )
            region = Region(offset_x, y, content_width, content_height)
            add_placement(WidgetPlacement(region, widget, 0))
            y += region.height + margin
--- a/src/textual/message.py
+++ b/src/textual/message.py
@@ -1,10 +1,10 @@
 from __future__ import annotations

-from time import monotonic
 from typing import ClassVar

 import rich.repr

+from . import _clock
 from .case import camel_to_snake
 from ._types import MessageTarget

@@ -39,7 +39,7 @@ class Message:

        self.sender = sender
        self.name = camel_to_snake(self.__class__.__name__.replace("Message", ""))
-        self.time = monotonic()
+        self.time = _clock.get_time_no_wait()
        self._forwarded = False
        self._no_default_action = False
        self._stop_propagation = False
--- a/src/textual/message_pump.py
+++ b/src/textual/message_pump.py
@@ -12,7 +12,7 @@ from . import events
 from . import log
 from ._timer import Timer, TimerCallback
 from ._callback import invoke
-from ._context import active_app
+from ._context import active_app, NoActiveAppError
 from .message import Message
 from . import messages

@@ -74,8 +74,16 @@ class MessagePump:

    @property
    def app(self) -> "App":
-        """Get the current app."""
-        return active_app.get()
+        """
+        Get the current app.
+
+        Raises:
+            NoActiveAppError: if no active app could be found for the current asyncio context
+        """
+        try:
+            return active_app.get()
+        except LookupError:
+            raise NoActiveAppError()

    @property
    def is_parent_active(self):
@@ -152,7 +160,13 @@ class MessagePump:
        pause: bool = False,
    ) -> Timer:
        timer = Timer(
-            self, delay, self, name=name, callback=callback, repeat=0, pause=pause
+            self,
+            delay,
+            self,
+            name=name or f"set_timer#{Timer._timer_count}",
+            callback=callback,
+            repeat=0,
+            pause=pause,
        )
        self._child_tasks.add(timer.start())
        return timer
@@ -170,7 +184,7 @@ class MessagePump:
            self,
            interval,
            self,
-            name=name,
+            name=name or f"set_interval#{Timer._timer_count}",
            callback=callback,
            repeat=repeat or None,
            pause=pause,
--- a/src/textual/screen.py
+++ b/src/textual/screen.py
@@ -1,5 +1,7 @@
 from __future__ import annotations

+import sys
+
 from rich.console import RenderableType
 import rich.repr
 from rich.style import Style
@@ -12,6 +14,14 @@ from ._compositor import Compositor, MapGeometry
 from .reactive import Reactive
 from .widget import Widget

+if sys.version_info >= (3, 8):
+    from typing import Final
+else:
+    from typing_extensions import Final
+
+# Screen updates will be batched so that they don't happen more often than 20 times per second:
+UPDATE_PERIOD: Final = 1 / 20
+

@rich.repr.auto
 class Screen(Widget):
@@ -158,7 +168,9 @@ class Screen(Widget):
        self.check_idle()

    def on_mount(self, event: events.Mount) -> None:
-        self._update_timer = self.set_interval(1 / 20, self._on_update, pause=True)
+        self._update_timer = self.set_interval(
+            UPDATE_PERIOD, self._on_update, name="screen_update", pause=True
+        )

    async def on_resize(self, event: events.Resize) -> None:
        self.size_updated(event.size, event.virtual_size, event.container_size)
--- a/src/textual/widget.py
+++ b/src/textual/widget.py
@@ -184,7 +184,10 @@ class Widget(DOMNode):
            int: The optimal width of the content.
        """
        if self.is_container:
-            return self.layout.get_content_width(self, container, viewport)
+            return (
+                self.layout.get_content_width(self, container, viewport)
+                + self.scrollbar_width
+            )

        cache_key = container.width
        if self._content_width_cache[0] == cache_key:
@@ -214,11 +217,14 @@ class Widget(DOMNode):
        """
        if self.is_container:
            assert self.layout is not None
-            height = self.layout.get_content_height(
-                self,
-                container,
-                viewport,
-                width,
+            height = (
+                self.layout.get_content_height(
+                    self,
+                    container,
+                    viewport,
+                    width,
+                )
+                + self.scrollbar_height
            )
        else:
            cache_key = width
@@ -255,11 +261,21 @@ class Widget(DOMNode):

    @property
    def max_scroll_x(self) -> float:
-        return max(0, self.virtual_size.width - self.container_size.width)
+        """The maximum value of `scroll_x`."""
+        return max(
+            0,
+            self.virtual_size.width - self.container_size.width + self.scrollbar_width,
+        )

    @property
    def max_scroll_y(self) -> float:
-        return max(0, self.virtual_size.height - self.container_size.height)
+        """The maximum value of `scroll_y`."""
+        return max(
+            0,
+            self.virtual_size.height
+            - self.container_size.height
+            + self.scrollbar_height,
+        )

    @property
    def vertical_scrollbar(self) -> ScrollBar:
@@ -335,12 +351,27 @@ class Widget(DOMNode):
            tuple[bool, bool]: A tuple of (<vertical scrollbar enabled>, <horizontal scrollbar enabled>)

        """
-        if self.layout is None:
+        if not self.is_container:
            return False, False

        enabled = self.show_vertical_scrollbar, self.show_horizontal_scrollbar
        return enabled

+    @property
+    def scrollbar_dimensions(self) -> tuple[int, int]:
+        """Get the size of any scrollbars on the widget"""
+        return (int(self.show_horizontal_scrollbar), int(self.show_vertical_scrollbar))
+
+    @property
+    def scrollbar_width(self) -> int:
+        """Get the width used by the *vertical* scrollbar."""
+        return int(self.show_vertical_scrollbar)
+
+    @property
+    def scrollbar_height(self) -> int:
+        """Get the height used by the *horizontal* scrollbar."""
+        return int(self.show_horizontal_scrollbar)
+
    def set_dirty(self) -> None:
        """Set the Widget as 'dirty' (requiring re-render)."""
        self._dirty_regions.clear()
@@ -366,9 +397,10 @@ class Widget(DOMNode):
            bool: True if the scroll position changed, otherwise False.
        """
        scrolled_x = scrolled_y = False
-
        if animate:
            # TODO: configure animation speed
+            if duration is None and speed is None:
+                speed = 50
            if x is not None:
                self.scroll_target_x = x
                if x != self.scroll_x:
@@ -915,8 +947,6 @@ class Widget(DOMNode):

    def on_descendant_focus(self, event: events.DescendantFocus) -> None:
        self.descendant_has_focus = True
-        if self.is_container and isinstance(event.sender, Widget):
-            self.scroll_to_widget(event.sender, animate=True)

    def on_descendant_blur(self, event: events.DescendantBlur) -> None:
        self.descendant_has_focus = False
--- a/tests/test_animator.py
+++ b/tests/test_animator.py
@@ -177,12 +177,12 @@ class MockAnimator(Animator):
        self._time = 0.0
        self._on_animation_frame_called = False

-    def get_time(self):
-        return self._time
-
    def on_animation_frame(self):
        self._on_animation_frame_called = True

+    def _get_time(self):
+        return self._time
+

 def test_animator():

@@ -245,10 +245,3 @@ def test_bound_animator():
        easing=EASING[DEFAULT_EASING],
    )
    assert animator._animations[(id(animate_test), "foo")] == expected
-
-
-def test_animator_get_time():
-    target = Mock()
-    animator = Animator(target)
-    assert isinstance(animator.get_time(), float)
-    assert animator.get_time() <= animator.get_time()
--- a/tests/test_integration_layout.py
+++ b/tests/test_integration_layout.py
@@ -1,5 +1,4 @@
 from __future__ import annotations
-import asyncio
 from typing import cast, List

 import pytest
@@ -107,7 +106,6 @@ async def test_composition_of_vertical_container_with_children(
    expected_placeholders_size: tuple[int, int],
    expected_root_widget_virtual_size: tuple[int, int],
    expected_placeholders_offset_x: int,
-    event_loop: asyncio.AbstractEventLoop,
 ):
    class VerticalContainer(Widget):
        CSS = (
--- a/tests/test_integration_scrolling.py
+++ b/tests/test_integration_scrolling.py
@@ -1,17 +1,8 @@
 from __future__ import annotations
-
-import sys
 from typing import Sequence, cast

-if sys.version_info >= (3, 8):
-    from typing import Literal
-else:
-    from typing_extensions import Literal  # pragma: no cover
-
-
 import pytest

-from sandbox.vertical_container import VerticalContainer
 from tests.utilities.test_app import AppTest
 from textual.app import ComposeResult
 from textual.geometry import Size
@@ -21,7 +12,6 @@ from textual.widgets import Placeholder
 SCREEN_SIZE = Size(100, 30)


-@pytest.mark.skip("flaky test")
@pytest.mark.asyncio
@pytest.mark.integration_test  # this is a slow test, we may want to skip them in some contexts
@pytest.mark.parametrize(
@@ -32,23 +22,19 @@ SCREEN_SIZE = Size(100, 30)
        "scroll_to_animate",
        "waiting_duration",
        "last_screen_expected_placeholder_ids",
-        "last_screen_expected_out_of_viewport_placeholder_ids",
    ),
    (
-        [SCREEN_SIZE, 10, None, None, 0.01, (0, 1, 2, 3, 4), "others"],
-        [SCREEN_SIZE, 10, "placeholder_3", False, 0.01, (0, 1, 2, 3, 4), "others"],
-        [SCREEN_SIZE, 10, "placeholder_5", False, 0.01, (1, 2, 3, 4, 5), "others"],
-        [SCREEN_SIZE, 10, "placeholder_7", False, 0.01, (3, 4, 5, 6, 7), "others"],
-        [SCREEN_SIZE, 10, "placeholder_9", False, 0.01, (5, 6, 7, 8, 9), "others"],
+        [SCREEN_SIZE, 10, None, None, 0.01, (0, 1, 2, 3, 4)],
+        [SCREEN_SIZE, 10, "placeholder_3", False, 0.01, (0, 1, 2, 3, 4)],
+        [SCREEN_SIZE, 10, "placeholder_5", False, 0.01, (1, 2, 3, 4, 5)],
+        [SCREEN_SIZE, 10, "placeholder_7", False, 0.01, (3, 4, 5, 6, 7)],
+        [SCREEN_SIZE, 10, "placeholder_9", False, 0.01, (5, 6, 7, 8, 9)],
        # N.B. Scroll duration is hard-coded to 0.2 in the `scroll_to_widget` method atm
        # Waiting for this duration should allow us to see the scroll finished:
-        [SCREEN_SIZE, 10, "placeholder_9", True, 0.21, (5, 6, 7, 8, 9), "others"],
+        [SCREEN_SIZE, 10, "placeholder_9", True, 0.21, (5, 6, 7, 8, 9)],
        # After having waited for approximately half of the scrolling duration, we should
        # see the middle Placeholders as we're scrolling towards the last of them.
-        # The state of the screen at this "halfway there" timing looks to not be deterministic though,
-        # depending on the environment - so let's only assert stuff for the middle placeholders
-        # and the first and last ones, but without being too specific about the others:
-        [SCREEN_SIZE, 10, "placeholder_9", True, 0.1, (5, 6, 7), (1, 2, 9)],
+        [SCREEN_SIZE, 10, "placeholder_9", True, 0.1, (4, 5, 6, 7, 8)],
    ),
 )
 async def test_scroll_to_widget(
@@ -58,9 +44,19 @@ async def test_scroll_to_widget(
    scroll_to_placeholder_id: str | None,
    waiting_duration: float | None,
    last_screen_expected_placeholder_ids: Sequence[int],
-    last_screen_expected_out_of_viewport_placeholder_ids: Sequence[int]
-    | Literal["others"],
 ):
+    class VerticalContainer(Widget):
+        CSS = """
+        VerticalContainer {
+            layout: vertical;
+            overflow: hidden auto;
+        }
+        VerticalContainer Placeholder {
+            margin: 1 0;
+            height: 5;
+        }
+        """
+
    class MyTestApp(AppTest):
        CSS = """
        Placeholder {
@@ -78,7 +74,7 @@ async def test_scroll_to_widget(

    app = MyTestApp(size=screen_size, test_name="scroll_to_widget")

-    async with app.in_running_state(waiting_duration_post_yield=waiting_duration or 0):
+    async with app.in_running_state(waiting_duration_after_yield=waiting_duration or 0):
        if scroll_to_placeholder_id:
            target_widget_container = cast(Widget, app.query("#root").first())
            target_widget = cast(
@@ -97,21 +93,21 @@ async def test_scroll_to_widget(

    # Let's start by checking placeholders that should be visible:
    for placeholder_id in last_screen_expected_placeholder_ids:
-        assert (
-            placeholders_visibility_by_id[placeholder_id] is True
-        ), f"Placeholder '{placeholder_id}' should be visible but isn't"
+        assert placeholders_visibility_by_id[placeholder_id] is True, (
+            f"Placeholder '{placeholder_id}' should be visible but isn't"
+            f" :: placeholders_visibility_by_id={placeholders_visibility_by_id}"
+        )

    # Ok, now for placeholders that should *not* be visible:
-    if last_screen_expected_out_of_viewport_placeholder_ids == "others":
-        # We're simply going to check that all the placeholders that are not in
-        # `last_screen_expected_placeholder_ids` are not on the screen:
-        last_screen_expected_out_of_viewport_placeholder_ids = sorted(
-            tuple(
-                set(range(placeholders_count))
-                - set(last_screen_expected_placeholder_ids)
-            )
+    # We're simply going to check that all the placeholders that are not in
+    # `last_screen_expected_placeholder_ids` are not on the screen:
+    last_screen_expected_out_of_viewport_placeholder_ids = sorted(
+        tuple(
+            set(range(placeholders_count)) - set(last_screen_expected_placeholder_ids)
        )
+    )
    for placeholder_id in last_screen_expected_out_of_viewport_placeholder_ids:
-        assert (
-            placeholders_visibility_by_id[placeholder_id] is False
-        ), f"Placeholder '{placeholder_id}' should not be visible but is"
+        assert placeholders_visibility_by_id[placeholder_id] is False, (
+            f"Placeholder '{placeholder_id}' should not be visible but is"
+            f" :: placeholders_visibility_by_id={placeholders_visibility_by_id}"
+        )
--- a/tests/utilities/test_app.py
+++ b/tests/utilities/test_app.py
@@ -3,16 +3,20 @@ from __future__ import annotations
 import asyncio
 import contextlib
 import io
+from math import ceil
 from pathlib import Path
-from typing import AsyncContextManager, cast
+from time import monotonic
+from typing import AsyncContextManager, cast, ContextManager
+from unittest import mock

 from rich.console import Console

 from textual import events, errors
-from textual.app import App, ReturnType, ComposeResult
+from textual._clock import _Clock
+from textual.app import App, ComposeResult, WINDOWS
+from textual._context import active_app
 from textual.driver import Driver
-from textual.geometry import Size
-
+from textual.geometry import Size, Region

 # N.B. These classes would better be named TestApp/TestConsole/TestDriver/etc,
 # but it makes pytest emit warning as it will try to collect them as classes containing test cases :-/
@@ -61,33 +65,42 @@ class AppTest(App):
    def in_running_state(
        self,
        *,
-        waiting_duration_after_initialisation: float = 0.1,
-        waiting_duration_post_yield: float = 0,
-    ) -> AsyncContextManager:
+        time_mocking_ticks_granularity_fps: int = 60,  # i.e. when moving forward by 1 second we'll do it though 60 ticks
+        waiting_duration_after_initialisation: float = 1,
+        waiting_duration_after_yield: float = 0,
+    ) -> AsyncContextManager[ClockMock]:
        async def run_app() -> None:
            await self.process_messages()

        @contextlib.asynccontextmanager
        async def get_running_state_context_manager():
-            self._set_active()
-            run_task = asyncio.create_task(run_app())
-            timeout_before_yielding_task = asyncio.create_task(
-                asyncio.sleep(waiting_duration_after_initialisation)
-            )
-            done, pending = await asyncio.wait(
-                (
-                    run_task,
-                    timeout_before_yielding_task,
-                ),
-                return_when=asyncio.FIRST_COMPLETED,
-            )
-            if run_task in done or run_task not in pending:
-                raise RuntimeError(
-                    "TestApp is no longer running after its initialization period"
-                )
-            yield
-            if waiting_duration_post_yield:
-                await asyncio.sleep(waiting_duration_post_yield)
+            with mock_textual_timers(
+                ticks_granularity_fps=time_mocking_ticks_granularity_fps
+            ) as clock_mock:
+                run_task = asyncio.create_task(run_app())
+
+                # We have to do this because `run_app()` is running in its own async task, and our test is going to
+                # run in this one - so the app must also be the active App in our current context:
+                self._set_active()
+
+                await clock_mock.advance_clock(waiting_duration_after_initialisation)
+                # make sure the App has entered its main loop at this stage:
+                assert self._driver is not None
+
+                await self.force_full_screen_update()
+
+                # And now it's time to pass the torch on to the test function!
+                # We provide the `move_clock_forward` function to it,
+                # so it can also do some time-based Textual stuff if it needs to:
+                yield clock_mock
+
+                await clock_mock.advance_clock(waiting_duration_after_yield)
+
+                # Make sure our screen is up-to-date before exiting the context manager,
+                # so tests using our `last_display_capture` for example can assert things on a fully refreshed screen:
+                await self.force_full_screen_update()
+
+            # End of simulated time: we just shut down ourselves:
            assert not run_task.done()
            await self.shutdown()

@@ -102,27 +115,10 @@ class AppTest(App):
        """Just a commodity shortcut for `async with app.in_running_state(): pass`, for simple cases"""
        async with self.in_running_state(
            waiting_duration_after_initialisation=waiting_duration_after_initialisation,
-            waiting_duration_post_yield=waiting_duration_before_shutdown,
+            waiting_duration_after_yield=waiting_duration_before_shutdown,
        ):
            pass

-    def run(self):
-        raise NotImplementedError(
-            "Use `async with my_test_app.in_running_state()` rather than `my_test_app.run()`"
-        )
-
-    @property
-    def total_capture(self) -> str | None:
-        return self.console.file.getvalue()
-
-    @property
-    def last_display_capture(self) -> str | None:
-        total_capture = self.total_capture
-        if not total_capture:
-            return None
-        last_display_start_index = total_capture.rindex(CLEAR_SCREEN_SEQUENCE)
-        return total_capture[last_display_start_index:]
-
    def get_char_at(self, x: int, y: int) -> str:
        """Get the character at the given cell or empty string

@@ -153,6 +149,54 @@ class AppTest(App):
                return segment.text[0]
        return ""

+    async def force_full_screen_update(
+        self, *, repaint: bool = True, layout: bool = True
+    ) -> None:
+        try:
+            screen = self.screen
+        except IndexError:
+            return  # the app may not have a screen yet
+
+        # We artificially tell the Compositor that the whole area should be refreshed
+        screen._compositor._dirty_regions = {
+            Region(0, 0, screen.size.width, screen.size.height),
+        }
+        screen.refresh(repaint=repaint, layout=layout)
+        # We also have to make sure we have at least one dirty widget, or `screen._on_update()` will early return:
+        screen._dirty_widgets.add(screen)
+        screen._on_update()
+
+        await let_asyncio_process_some_events()
+
+    def on_exception(self, error: Exception) -> None:
+        # In tests we want the errors to be raised, rather than printed to a Console
+        raise error
+
+    def run(self):
+        raise NotImplementedError(
+            "Use `async with my_test_app.in_running_state()` rather than `my_test_app.run()`"
+        )
+
+    @property
+    def active_app(self) -> App | None:
+        return active_app.get()
+
+    @property
+    def total_capture(self) -> str | None:
+        return self.console.file.getvalue()
+
+    @property
+    def last_display_capture(self) -> str | None:
+        total_capture = self.total_capture
+        if not total_capture:
+            return None
+        screen_captures = total_capture.split(CLEAR_SCREEN_SEQUENCE)
+        for single_screen_capture in reversed(screen_captures):
+            if len(single_screen_capture) > 30:
+                # let's return the last occurrence of a screen that seem to be properly "fully-paint"
+                return single_screen_capture
+        return None
+
    @property
    def console(self) -> ConsoleTest:
        return self._console
@@ -207,3 +251,111 @@ class DriverTest(Driver):

    def stop_application_mode(self) -> None:
        pass
+
+
+# It seems that we have to give _way more_ time to `asyncio` on Windows in order to see our different awaiters
+# properly triggered when we pause our own "move clock forward" loop.
+# It could be caused by the fact that the time resolution for `asyncio` on this platform seems rather low:
+# > The resolution of the monotonic clock on Windows is usually around 15.6 msec.
+# > The best resolution is 0.5 msec.
+# @link https://docs.python.org/3/library/asyncio-platforms.html:
+ASYNCIO_EVENTS_PROCESSING_REQUIRED_PERIOD = 0.025 if WINDOWS else 0.005
+
+
+async def let_asyncio_process_some_events() -> None:
+    await asyncio.sleep(ASYNCIO_EVENTS_PROCESSING_REQUIRED_PERIOD)
+
+
+class ClockMock(_Clock):
+    # To avoid issues with floats we will store the current time as an integer internally.
+    # Tenths of microseconds should be a good enough granularity:
+    TIME_RESOLUTION = 10_000_000
+
+    def __init__(
+        self,
+        *,
+        ticks_granularity_fps: int = 60,
+    ):
+        self._ticks_granularity_fps = ticks_granularity_fps
+        self._single_tick_duration = int(self.TIME_RESOLUTION / ticks_granularity_fps)
+        self._start_time: int = -1
+        self._current_time: int = -1
+        # For each call to our `sleep` method we will store an asyncio.Event
+        # and the time at which we should trigger it:
+        self._pending_sleep_events: dict[int, list[asyncio.Event]] = {}
+
+    def get_time_no_wait(self) -> float:
+        if self._current_time == -1:
+            self._start_clock()
+
+        return self._current_time / self.TIME_RESOLUTION
+
+    async def sleep(self, seconds: float) -> None:
+        event = asyncio.Event()
+        internal_waiting_duration = int(seconds * self.TIME_RESOLUTION)
+        target_event_monotonic_time = self._current_time + internal_waiting_duration
+        self._pending_sleep_events.setdefault(target_event_monotonic_time, []).append(
+            event
+        )
+        # Ok, let's wait for this Event
+        # (which can only be "unlocked" by calls to `advance_clock()`)
+        await event.wait()
+
+    async def advance_clock(self, seconds: float) -> None:
+        """
+        Artificially advance the Textual clock forward.
+
+        Args:
+            seconds: for each second we will artificially tick `ticks_granularity_fps` times
+        """
+        if self._current_time == -1:
+            self._start_clock()
+
+        ticks_count = ceil(seconds * self._ticks_granularity_fps)
+        activated_timers_count_total = 0  # useful when debugging this code :-)
+        for tick_counter in range(ticks_count):
+            self._current_time += self._single_tick_duration
+            activated_timers_count = self._check_sleep_timers_to_activate()
+            activated_timers_count_total += activated_timers_count
+            # Now that we likely unlocked some occurrences of `await sleep(duration)`,
+            # let's give an opportunity to asyncio-related stuff to happen:
+            if activated_timers_count:
+                await let_asyncio_process_some_events()
+
+        await let_asyncio_process_some_events()
+
+    def _start_clock(self) -> None:
+        # N.B. `start_time` is not actually used, but it is useful to have when we set breakpoints there :-)
+        self._start_time = self._current_time = int(monotonic() * self.TIME_RESOLUTION)
+
+    def _check_sleep_timers_to_activate(self) -> int:
+        activated_timers_count = 0
+        activated_events_times_to_clear: list[int] = []
+        for (monotonic_time, target_events) in self._pending_sleep_events.items():
+            if self._current_time < monotonic_time:
+                continue  # not time for you yet, dear awaiter...
+            # Right, let's release these waiting events!
+            for event in target_events:
+                event.set()
+            activated_timers_count += len(target_events)
+            # ...and let's mark it for removal:
+            activated_events_times_to_clear.append(monotonic_time)
+
+        if activated_events_times_to_clear:
+            for event_time_to_clear in activated_events_times_to_clear:
+                del self._pending_sleep_events[event_time_to_clear]
+
+        return activated_timers_count
+
+
+def mock_textual_timers(
+    *,
+    ticks_granularity_fps: int = 60,
+) -> ContextManager[ClockMock]:
+    @contextlib.contextmanager
+    def mock_textual_timers_context_manager():
+        clock_mock = ClockMock(ticks_granularity_fps=ticks_granularity_fps)
+        with mock.patch("textual._clock._clock", new=clock_mock):
+            yield clock_mock
+
+    return mock_textual_timers_context_manager()