Merge branch 'main' into review-styles-reference

.pre-commit-config.yaml

@@ -12,5 +12,4 @@ repos:
     rev: 22.10.0
     hooks:
     -   id: black
-        exclude: ^tests/
 exclude: ^tests/snapshot_tests

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - `MouseScrollUp` and `MouseScrollDown` now inherit from `MouseEvent` and have attached modifier keys. https://github.com/Textualize/textual/pull/1458
 - Fail-fast and print pretty tracebacks for Widget compose errors https://github.com/Textualize/textual/pull/1505
+- Added Widget._refresh_scroll to avoid expensive layout when scrolling https://github.com/Textualize/textual/pull/1524
 
 ### Fixed
 

docs/blog/posts/looking-for-help.md

@@ -0,0 +1,323 @@
+---
+draft: false
+date: 2023-01-09
+categories:
+  - DevLog
+authors:
+  - davep
+---
+
+# So you're looking for a wee bit of Textual help...
+
+## Introduction
+
+!!! quote
+
+    Patience, Highlander. You have done well. But it'll take time. You are
+    generations being born and dying. You are at one with all living things.
+    Each man's thoughts and dreams are yours to know. You have power beyond
+    imagination. Use it well, my friend. Don't lose your head.
+
+    <cite>Juan Sánchez Villalobos Ramírez, Chief metallurgist to King Charles V of Spain</cite>
+
+As of the time of writing, I'm a couple or so days off having been with
+Textualize for 3 months. It's been fun, and educational, and every bit as
+engaging as I'd hoped, and more. One thing I hadn't quite prepared for
+though, but which I really love, is how so many other people are learning
+Textual along with me.
+
+<!-- more -->
+
+Even in those three months the library has changed and expanded quite a lot,
+and it continues to do so. Meanwhile, more people are turning up and using
+the framework; you can see this online in social media, blogs and of course
+[in the ever-growing list of projects on GitHub which depend on
+Textual](https://github.com/Textualize/textual/network/dependents).
+
+This inevitably means there's a lot of people getting to grips with a new
+tool, and one that is still a bit of a moving target. This in turn means
+lots of people are coming to us to get help.
+
+As I've watched this happen I've noticed a few patterns emerging. Some of
+these good or neutral, some... let's just say not really beneficial to those
+seeking the help, or to those trying to provide the help. So I wanted to
+write a little bit about the different ways you can get help with Textual
+and your Textual-based projects, and to also try and encourage people to
+take the most helpful and positive approach to getting that help.
+
+Now, before I go on, I want to make something *very* clear: I'm writing this
+as an individual. This is my own personal view, and my own advice from me to
+anyone who wishes to take it. It's not Textual (the project) or Textualize
+(the company) policy, rules or guidelines. This is just some ageing hacker's
+take on how best to go about asking for help, informed by years of asking
+for and also providing help in email, on Usenet, on forums, etc.
+
+Or, put another way: if what you read in here seems sensible to you, I
+figure we'll likely have already hit it off [over on
+GitHub](https://github.com/Textualize/textual) or in [the Discord
+server](https://discord.gg/Enf6Z3qhVr). ;-)
+
+## Where to go for help
+
+At this point this is almost a bit of an FAQ itself, so I thought I'd
+address it here: where's the best place to ask for help about Textual, and
+what's the difference between GitHub Issues, Discussions and our Discord
+server?
+
+I'd suggest thinking of them like this:
+
+### Discord
+
+You have a question, or need help with something, and perhaps you could do
+with a reply as soon as possible. But, and this is the **really important
+part**, it doesn't matter if you don't get a response. If you're in this
+situation then the Discord server is possibly a good place to start. If
+you're lucky someone will be hanging about who can help out.
+
+I can't speak for anyone else, but keep this in mind: when I look in on
+Discord I tend not to go scrolling back much to see if anything has been
+missed. If something catches my eye, I'll try and reply, but if it
+doesn't... well, it's mostly an instant chat thing so I don't dive too
+deeply back in time.
+
+!!! tip inline end "Going from Discord to a GitHub issue"
+
+    As a slight aside here: sometimes people will pop up in Discord, ask a
+    question about something that turns out looking like a bug, and that's
+    the last we hear of it. Please, please, **please**, if this happens, the
+    most helpful thing you can do is go raise an issue for us. It'll help us
+    to keep track of problems, it'll help get your problem fixed, it'll mean
+    everyone benefits.
+
+My own advice would be to treat Discord as an ephemeral resource. It happens
+in the moment but fades away pretty quickly. It's like knocking on a
+friend's door to see if they're in. If they're not in, you might leave them
+a note, which is sort of like going to...
+
+### GitHub
+
+On the other hand, if you have a question or need some help or something
+where you want to stand a good chance of the Textual developers (amongst
+others) seeing it and responding, I'd recommend that GitHub is the place to
+go. Dropping something into the discussions there, or leaving an issue,
+ensures it'll get seen. It won't get lost.
+
+As for which you should use -- a discussion or an issue -- I'd suggest this:
+if you need help with something, or you want to check your understanding of
+something, or you just want to be sure something is a problem before taking
+it further, a discussion might be the best thing. On the other hand, if
+you've got a clear bug or feature request on your hands, an issue makes a
+lot of sense.
+
+Don't worry if you're not sure which camp your question or whatever falls
+into though; go with what you think is right. There's no harm done either
+way (I may move an issue to a discussion first before replying, if it's
+really just a request for help -- but that's mostly so everyone can benefit
+from finding it in the right place later on down the line).
+
+## The dos and don'ts of getting help
+
+Now on to the fun part. This is where I get a bit preachy. Ish. Kinda. A
+little bit. Again, please remember, this isn't a set of rules, this isn't a
+set of official guidelines, this is just a bunch of *"if you want my advice,
+and I know you didn't ask but you've read this far so you actually sort of
+did don't say I didn't warn you!"* waffle.
+
+This isn't going to be an exhaustive collection, far from it. But I feel
+these are some important highlights.
+
+### Do...
+
+When looking for help, in any of the locations mentioned above, I'd totally
+encourage:
+
+#### Be clear and detailed
+
+Too much detail is almost always way better than not enough. *"My program
+didn't run"*, often even with some of the code supplied, is so much harder
+to help than *"I ran this code I'm posting here, and I expected this
+particular outcome, and I expected it because I'd read this particular thing
+in the docs and had comprehended it to mean this, but instead the outcome
+was this exception here, and I'm a bit stuck -- can someone offer some
+pointers?"*
+
+The former approach means there often ends up having to be a back and forth
+which can last a long time, and which can sometimes be frustrating for the
+person asking. Manage frustration: be clear, tell us everything you can.
+
+#### Say what resources you've used already
+
+If you've read the potions of the documentation that relate to what you're
+trying to do, it's going to be really helpful if you say so. If you don't,
+it might be assumed you haven't and you may end up being pointed at them.
+
+So, please, if you've checked the documentation, looked in the FAQ, done a
+search of past issues or discussions or perhaps even done a search on the
+Discord server... please say so.
+
+#### Be polite
+
+This one can go a long way when looking for help. Look, I get it,
+programming is bloody frustrating at times. We've all rage-quit some code at
+some point, I'm sure. It's likely going to be your moment of greatest
+frustration when you go looking for help. But if you turn up looking for
+help acting all grumpy and stuff it's not going to come over well. Folk are
+less likely to be motivated to lend a hand to someone who seems rather
+annoyed.
+
+If you throw in a please and thank-you here and there that makes it all the
+better.
+
+#### Fully consider the replies
+
+You could find yourself getting a reply that you're sure won't help at all.
+That's fair. But be sure to fully consider it first. Perhaps you missed the
+obvious along the way and this is 100% the course correction you'd
+unknowingly come looking for in the first place. Sure, the person replying
+might have totally misunderstood what was being asked, or might be giving a
+wrong answer (it me! I've totally done that and will again!), but even then
+a reply along the lines of *"I'm not sure that's what I'm looking for,
+because..."* gets everyone to the solution faster than *"lol nah"*.
+
+#### Entertain what might seem like odd questions
+
+Aye, I get it, being asked questions when you're looking for an *answer* can
+be a bit frustrating. But if you find yourself on the receiving end of a
+small series of questions about your question, keep this in mind: Textual is
+still rather new and still developing and it's possible that what you're
+trying to do isn't the correct way to do that thing. To the person looking
+to help you it may seem to them you have an [XY
+problem](https://en.wikipedia.org/wiki/XY_problem).
+
+Entertaining those questions might just get you to the real solution to your
+problem.
+
+#### Allow for language differences
+
+You don't need me to tell you that a project such as Textual has a global
+audience. With that rather obvious fact comes the other fact that we don't
+all share the same first language. So, please, as much as possible, try and
+allow for that. If someone is trying to help you out, and they make it clear
+they're struggling to follow you, keep this in mind.
+
+#### Acknowledge the answer
+
+I suppose this is a variation on "be polite" (really, a thanks can go a long
+way), but there's more to this than a friendly acknowledgement. If someone
+has gone to the trouble of offering some help, it's helpful to everyone who
+comes after you to acknowledge if it worked or not. That way a future
+help-seeker will know if the answer they're reading stands a chance of being
+the right one.
+
+#### Accept that Textual is zero-point software (right now)
+
+Of course the aim is to have every release of Textual be stable and useful,
+but things will break. So, please, do keep in mind things like:
+
+- Textual likely doesn't have your feature of choice just yet.
+- We might accidentally break something (perhaps pinning Textual and testing
+  each release is a good plan here?).
+- We might deliberately break something because we've decided to take a
+  particular feature or way of doing things in a better direction.
+
+Of course it can be a bit frustrating a times, but overall the aim is to
+have the best framework possible in the long run.
+
+### Don't...
+
+Okay, now for a bit of old-hacker finger-wagging. Here's a few things I'd
+personally discourage:
+
+#### Lack patience
+
+Sure, it can be annoying. You're in your flow, you've got a neat idea for a
+thing you want to build, you're stuck on one particular thing and you really
+need help right now! Thing is, that's unlikely to happen. Badgering
+individuals, or a whole resource, to reply right now, or complaining that
+it's been `$TIME_PERIOD` since you asked and nobody has replied... that's
+just going to make people less likely to reply.
+
+#### Unnecessarily tag individuals
+
+This one often goes hand in hand with the "lack patience" thing: Be it
+asking on Discord, or in GitHub issues, discussions or even PRs,
+unnecessarily tagging individuals is a bit rude. Speaking for myself and
+only myself: I *love* helping folk with Textual. If I could help everyone
+all the time the moment they have a problem, I would. But it doesn't work
+like that. There's any number of reasons I might not be responding to a
+particular request, including but not limited to (here I'm talking
+personally because I don't want to speak for anyone else, but I'm sure I'm
+not alone here):
+
+- I have a job. Sure, my job is (in part) Textual, but there's more to it
+  than that particular issue. I might be doing other stuff.
+- I have my own projects to work on too. I like coding for fun as well (or
+  writing preaching old dude blog posts like this I guess, but you get the
+  idea).
+- I actually have other interests outside of work hours so I might actually
+  be out doing a 10k in the local glen, or battling headcrabs in VR, or
+  something.
+- Housework. :-/
+
+You get the idea though. So while I'm off having a well-rounded life, it's
+not good to get unnecessarily intrusive alerts to something that either a)
+doesn't actually directly involve me or b) could wait.
+
+#### Seek personal support
+
+Again, I'm going to speak totally for myself here, but I also feel the
+general case is polite for all: there's a lot of good support resources
+available already; sending DMs on Discord or Twitter or in the Fediverse,
+looking for direct personal support, isn't really the best way to get help.
+Using the public/collective resources is absolutely the *best* way to get
+that help. Why's it a bad idea to dive into DMs? Here's some reasons I think
+it's not a good idea:
+
+- It's a variation on "unnecessarily tagging individuals".
+- You're short-changing yourself when it comes to getting help. If you ask
+  somewhere more public you're asking a much bigger audience, who
+  collectively have more time, more knowledge and more experience than a
+  single individual.
+- Following on from that, any answers can be (politely) fact-checked or
+  enhanced by that audience, resulting in a better chance of getting the
+  best help possible.
+- The next seeker-of-help gets to miss out on your question and the answer.
+  If asked and answered in public, it's a record that can help someone else
+  in the future.
+
+#### Doubt your ability or skill level
+
+I suppose this should really be phrased as a do rather than a don't, as here
+I want to encourage something positive. A few times I've helped people out
+who have been very apologetic about their questions being "noob" questions,
+or about how they're fairly new to Python, or programming in general.
+Really, please, don't feel the need to apologise and don't be ashamed of
+where you're at.
+
+If you've asked something that's obviously answered in the documentation,
+that's not a problem; you'll likely get pointed at the docs and it's what
+happens next that's the key bit. If the attitude is *"oh, cool, that's
+exactly what I needed to be reading, thanks!"* that's a really positive
+thing. The only time it's a problem is when there's a real reluctance to use
+the available resources. We've all seen that person somewhere at some point,
+right? ;-)
+
+Not knowing things [is totally cool](https://xkcd.com/1053/).
+
+## Conclusion
+
+So, that's my waffle over. As I said at the start: this is my own personal
+thoughts on how to get help with Textual, both as someone whose job it is to
+work on Textual and help people with Textual, and also as a FOSS advocate
+and supporter who can normally be found helping Textual users when he's not
+"on the clock" too.
+
+What I've written here isn't exhaustive. Neither is it novel. Plenty has
+been written on the general subject in the past, and I'm sure more will be
+written on the subject in the future. I do, however, feel that these are the
+most common things I notice. I'd say those dos and don'ts cover 90% of *"can
+I get some help?"* interactions; perhaps closer to 99%.
+
+Finally, and I think this is the most important thing to remember, the next
+time you are battling some issue while working with Textual: [don't lose
+your head](https://www.youtube.com/watch?v=KdYvKF9O7Y8)!

docs/help.md

@@ -6,11 +6,10 @@ If you need help with any aspect of Textual, let us know! We would be happy to h
 
 Report bugs via GitHub on the Textual [issues](https://github.com/Textualize/textual/issues) page. You can also post feature requests via GitHub issues, but see the [roadmap](./roadmap.md) first.
 
+## Help with using Textual
+
+You can seek help with using Textual [in the discussion area on GitHub](https://github.com/Textualize/textual/discussions).
+
 ## Discord Server
 
 For more realtime feedback or chat, join our discord server to connect with the [Textual community](https://discord.gg/Enf6Z3qhVr).
-
-## Forum
-
-Visit the [Textual forum](https://community.textualize.io/) for Textual (and Rich) discussions.
-

poetry.lock

@@ -182,7 +182,7 @@ test = ["flake8 (==3.7.8)", "hypothesis (==3.55.3)"]
 
 [[package]]
 name = "coverage"
-version = "7.0.1"
+version = "7.0.3"
 description = "Code coverage measurement for Python"
 category = "dev"
 optional = false
@@ -269,7 +269,7 @@ typing-extensions = {version = ">=3.7.4.3", markers = "python_version < \"3.8\""
 
 [[package]]
 name = "griffe"
-version = "0.25.2"
+version = "0.25.3"
 description = "Signatures for entire Python programs. Extract the structure, the frame, the skeleton of your project, to generate API documentation or find breaking changes in your API."
 category = "dev"
 optional = false
@@ -313,7 +313,7 @@ socks = ["socksio (>=1.0.0,<2.0.0)"]
 
 [[package]]
 name = "httpx"
-version = "0.23.2"
+version = "0.23.3"
 description = "The next generation HTTP client."
 category = "dev"
 optional = false
@@ -333,7 +333,7 @@ socks = ["socksio (>=1.0.0,<2.0.0)"]
 
 [[package]]
 name = "identify"
-version = "2.5.11"
+version = "2.5.12"
 description = "File identification library for Python"
 category = "dev"
 optional = false
@@ -524,7 +524,7 @@ python-legacy = ["mkdocstrings-python-legacy (>=0.2.1)"]
 
 [[package]]
 name = "mkdocstrings-python"
-version = "0.8.2"
+version = "0.8.3"
 description = "A Python handler for mkdocstrings."
 category = "dev"
 optional = false
@@ -1178,57 +1178,57 @@ commonmark = [
     {file = "commonmark-0.9.1.tar.gz", hash = "sha256:452f9dc859be7f06631ddcb328b6919c67984aca654e5fefb3914d54691aed60"},
 ]
 coverage = [
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 mkdocstrings-python = [
-    {file = "mkdocstrings-python-0.8.2.tar.gz", hash = "sha256:b22528b7a7a0589d007eced019d97ad14de4eba4b2b9ba6a013bb66edc74ab43"},
+    {file = "mkdocstrings-python-0.8.3.tar.gz", hash = "sha256:9ae473f6dc599339b09eee17e4d2b05d6ac0ec29860f3fc9b7512d940fc61adf"},
-    {file = "mkdocstrings_python-0.8.2-py3-none-any.whl", hash = "sha256:213d9592e66e084a9bd2fa4956d6294a3487c6dc9cc45164058d6317249b7b6e"},
+    {file = "mkdocstrings_python-0.8.3-py3-none-any.whl", hash = "sha256:4e6e1cd6f37a785de0946ced6eb846eb2f5d891ac1cc2c7b832943d3529087a7"},
 ]
 msgpack = [
     {file = "msgpack-1.0.4-cp310-cp310-macosx_10_9_universal2.whl", hash = "sha256:4ab251d229d10498e9a2f3b1e68ef64cb393394ec477e3370c457f9430ce9250"},

src/textual/_compositor.py

@@ -253,8 +253,8 @@ class Compositor:
         # Keep a copy of the old map because we're going to compare it with the update
         old_map = self.map.copy()
         old_widgets = old_map.keys()
-        map, widgets = self._arrange_root(parent, size)
 
+        map, widgets = self._arrange_root(parent, size)
         new_widgets = map.keys()
 
         # Newly visible widgets
@@ -266,20 +266,21 @@ class Compositor:
         self.map = map
         self.widgets = widgets
 
-        screen = size.region
+        # Contains widgets + geometry for every widget that changed (added, removed, or updated)
+        changes = map.items() ^ old_map.items()
+
+        # Widgets in both new and old
+        common_widgets = old_widgets & new_widgets
 
         # Widgets with changed size
         resized_widgets = {
             widget
-            for widget, (region, *_) in map.items()
+            for widget, (region, *_) in changes
-            if widget in old_widgets and old_map[widget].region.size != region.size
+            if (widget in common_widgets and old_map[widget].region[2:] != region[2:])
         }
 
-        # Gets pairs of tuples of (Widget, MapGeometry) which have changed
+        screen_region = size.region
-        # i.e. if something is moved / deleted / added
+        if screen_region not in self._dirty_regions:
-
-        if screen not in self._dirty_regions:
-            changes = map.items() ^ old_map.items()
             regions = {
                 region
                 for region in (
@@ -346,6 +347,7 @@ class Compositor:
             layer_order: int,
             clip: Region,
             visible: bool,
+            _MapGeometry=MapGeometry,
         ) -> None:
             """Called recursively to place a widget and its children in the map.
 
@@ -392,50 +394,57 @@ class Compositor:
                     )
                     widgets.update(arranged_widgets)
 
-                    # An offset added to all placements
+                    if placements:
-                    placement_offset = container_region.offset
+                        # An offset added to all placements
-                    placement_scroll_offset = placement_offset - widget.scroll_offset
+                        placement_offset = container_region.offset
+                        placement_scroll_offset = (
+                            placement_offset - widget.scroll_offset
+                        )
 
-                    _layers = widget.layers
+                        _layers = widget.layers
-                    layers_to_index = {
+                        layers_to_index = {
-                        layer_name: index for index, layer_name in enumerate(_layers)
+                            layer_name: index
-                    }
+                            for index, layer_name in enumerate(_layers)
-                    get_layer_index = layers_to_index.get
+                        }
+                        get_layer_index = layers_to_index.get
 
-                    # Add all the widgets
+                        # Add all the widgets
-                    for sub_region, margin, sub_widget, z, fixed in reversed(
+                        for sub_region, margin, sub_widget, z, fixed in reversed(
-                        placements
+                            placements
-                    ):
+                        ):
-                        # Combine regions with children to calculate the "virtual size"
+                            # Combine regions with children to calculate the "virtual size"
-                        if fixed:
+                            if fixed:
-                            widget_region = sub_region + placement_offset
+                                widget_region = sub_region + placement_offset
-                        else:
+                            else:
-                            total_region = total_region.union(
+                                total_region = total_region.union(
-                                sub_region.grow(spacing + margin)
+                                    sub_region.grow(spacing + margin)
+                                )
+                                widget_region = sub_region + placement_scroll_offset
+
+                            widget_order = (
+                                *order,
+                                get_layer_index(sub_widget.layer, 0),
+                                z,
+                                layer_order,
                             )
-                            widget_region = sub_region + placement_scroll_offset
 
-                        widget_order = order + (
+                            add_widget(
-                            (get_layer_index(sub_widget.layer, 0), z, layer_order),
+                                sub_widget,
-                        )
+                                sub_region,
-
+                                widget_region,
-                        add_widget(
+                                widget_order,
-                            sub_widget,
+                                layer_order,
-                            sub_region,
+                                sub_clip,
-                            widget_region,
+                                visible,
-                            widget_order,
+                            )
-                            layer_order,
+                            layer_order -= 1
-                            sub_clip,
-                            visible,
-                        )
-                        layer_order -= 1
 
                 if visible:
                     # Add any scrollbars
                     for chrome_widget, chrome_region in widget._arrange_scrollbars(
                         container_region
                     ):
-                        map[chrome_widget] = MapGeometry(
+                        map[chrome_widget] = _MapGeometry(
                             chrome_region + layout_offset,
                             order,
                             clip,
@@ -444,7 +453,7 @@ class Compositor:
                             chrome_region,
                         )
 
-                    map[widget] = MapGeometry(
+                    map[widget] = _MapGeometry(
                         region + layout_offset,
                         order,
                         clip,
@@ -455,7 +464,7 @@ class Compositor:
 
             elif visible:
                 # Add the widget to the map
-                map[widget] = MapGeometry(
+                map[widget] = _MapGeometry(
                     region + layout_offset,
                     order,
                     clip,

src/textual/_segment_tools.py

@@ -14,6 +14,56 @@ from .css.types import AlignHorizontal, AlignVertical
 from .geometry import Size
 
 
+class NoCellPositionForIndex(Exception):
+    pass
+
+
+def index_to_cell_position(segments: Iterable[Segment], index: int) -> int:
+    """Given a character index, return the cell position of that character within
+    an Iterable of Segments. This is the sum of the cell lengths of all the characters
+    *before* the character at `index`.
+
+    Args:
+        segments (Iterable[Segment]): The segments to find the cell position within.
+        index (int): The index to convert into a cell position.
+
+    Returns:
+        int: The cell position of the character at `index`.
+
+    Raises:
+        NoCellPositionForIndex: If the supplied index doesn't fall within the given segments.
+    """
+    if not segments:
+        raise NoCellPositionForIndex
+
+    if index == 0:
+        return 0
+
+    cell_position_end = 0
+    segment_length = 0
+    segment_end_index = 0
+    segment_cell_length = 0
+    text = ""
+    iter_segments = iter(segments)
+    try:
+        while segment_end_index < index:
+            segment = next(iter_segments)
+            text = segment.text
+            segment_length = len(text)
+            segment_cell_length = cell_len(text)
+            cell_position_end += segment_cell_length
+            segment_end_index += segment_length
+    except StopIteration:
+        raise NoCellPositionForIndex
+
+    # Check how far into this segment the target index is
+    segment_index_start = segment_end_index - segment_length
+    index_within_segment = index - segment_index_start
+    segment_cell_start = cell_position_end - segment_cell_length
+
+    return segment_cell_start + cell_len(text[:index_within_segment])
+
+
 def line_crop(
     segments: list[Segment], start: int, end: int, total: int
 ) -> list[Segment]:

src/textual/app.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
 import asyncio
+from concurrent.futures import Future
+from functools import partial
 import inspect
 import io
 import os
@@ -18,6 +20,7 @@ from time import perf_counter
 from typing import (
     TYPE_CHECKING,
     Any,
+    Awaitable,
     Callable,
     Generic,
     Iterable,
@@ -206,6 +209,8 @@ class _WriterThread(threading.Thread):
 
 CSSPathType = Union[str, PurePath, List[Union[str, PurePath]], None]
 
+CallThreadReturnType = TypeVar("CallThreadReturnType")
+
 
 @rich.repr.auto
 class App(Generic[ReturnType], DOMNode):
@@ -353,6 +358,8 @@ class App(Generic[ReturnType], DOMNode):
             else:
                 self.devtools = DevtoolsClient()
 
+        self._loop: asyncio.AbstractEventLoop | None = None
+        self._thread_id: int = 0
         self._return_value: ReturnType | None = None
         self._exit = False
 
@@ -604,6 +611,51 @@ class App(Generic[ReturnType], DOMNode):
         except Exception as error:
             self._handle_exception(error)
 
+    def call_from_thread(
+        self,
+        callback: Callable[..., CallThreadReturnType | Awaitable[CallThreadReturnType]],
+        *args,
+        **kwargs,
+    ) -> CallThreadReturnType:
+        """Run a callback from another thread.
+
+        Like asyncio apps in general, Textual apps are not thread-safe. If you call methods
+        or set attributes on Textual objects from a thread, you may get unpredictable results.
+
+        This method will ensure that your code is ran within the correct context.
+
+        Args:
+            callback (Callable): A callable to run.
+            *args: Arguments to the callback.
+            **kwargs: Keyword arguments for the callback.
+
+        Raises:
+            RuntimeError: If the app isn't running or if this method is called from the same
+                thread where the app is running.
+        """
+
+        if self._loop is None:
+            raise RuntimeError("App is not running")
+
+        if self._thread_id == threading.get_ident():
+            raise RuntimeError(
+                "The `call_from_thread` method must run in a different thread from the app"
+            )
+
+        callback_with_args = partial(callback, *args, **kwargs)
+
+        async def run_callback() -> CallThreadReturnType:
+            """Run the callback, set the result or error on the future."""
+            self._set_active()
+            return await invoke(callback_with_args)
+
+        # Post the message to the main loop
+        future: Future[CallThreadReturnType] = asyncio.run_coroutine_threadsafe(
+            run_callback(), loop=self._loop
+        )
+        result = future.result()
+        return result
+
     def action_toggle_dark(self) -> None:
         """Action to toggle dark mode."""
         self.dark = not self.dark
@@ -874,11 +926,17 @@ class App(Generic[ReturnType], DOMNode):
 
         async def run_app() -> None:
             """Run the app."""
-            await self.run_async(
+            self._loop = asyncio.get_running_loop()
-                headless=headless,
+            self._thread_id = threading.get_ident()
-                size=size,
+            try:
-                auto_pilot=auto_pilot,
+                await self.run_async(
-            )
+                    headless=headless,
+                    size=size,
+                    auto_pilot=auto_pilot,
+                )
+            finally:
+                self._loop = None
+                self._thread_id = 0
 
         if _ASYNCIO_GET_EVENT_LOOP_IS_DEPRECATED:
             # N.B. This doesn't work with Python<3.10, as we end up with 2 event loops:

src/textual/geometry.py

@@ -684,6 +684,7 @@ class Region(NamedTuple):
         )
         return new_region
 
+    @lru_cache(maxsize=4096)
     def grow(self, margin: tuple[int, int, int, int]) -> Region:
         """Grow a region by adding spacing.
 
@@ -704,6 +705,7 @@ class Region(NamedTuple):
             height=max(0, height + top + bottom),
         )
 
+    @lru_cache(maxsize=4096)
     def shrink(self, margin: tuple[int, int, int, int]) -> Region:
         """Shrink a region by subtracting spacing.
 
@@ -713,7 +715,8 @@ class Region(NamedTuple):
         Returns:
             Region: The new, smaller region.
         """
-
+        if not any(margin):
+            return self
         top, right, bottom, left = margin
         x, y, width, height = self
         return Region(

src/textual/strip.py

@@ -10,6 +10,7 @@ from rich.style import Style
 
 from ._cache import FIFOCache
 from ._filter import LineFilter
+from ._segment_tools import index_to_cell_position
 
 
 @rich.repr.auto
@@ -68,6 +69,19 @@ class Strip:
         """
         return [cls(segments, cell_length) for segments in lines]
 
+    def index_to_cell_position(self, index: int) -> int:
+        """Given a character index, return the cell position of that character.
+        This is the sum of the cell lengths of all the characters *before* the character
+        at `index`.
+
+        Args:
+            index (int): The index to convert.
+
+        Returns:
+            int: The cell position of the character at `index`.
+        """
+        return index_to_cell_position(self._segments, index)
+
     @property
     def cell_length(self) -> int:
         """Get the number of cells required to render this object."""

src/textual/widget.py

@@ -802,15 +802,17 @@ class Widget(DOMNode):
         if self.auto_links:
             self.highlight_link_id = hover_style.link_id
 
-    def watch_scroll_x(self, new_value: float) -> None:
+    def watch_scroll_x(self, old_value: float, new_value: float) -> None:
         if self.show_horizontal_scrollbar:
-            self.horizontal_scrollbar.position = int(new_value)
+            self.horizontal_scrollbar.position = round(new_value)
-            self.refresh(layout=True, repaint=False)
+            if round(old_value) != round(new_value):
+                self._refresh_scroll()
 
-    def watch_scroll_y(self, new_value: float) -> None:
+    def watch_scroll_y(self, old_value: float, new_value: float) -> None:
         if self.show_vertical_scrollbar:
-            self.vertical_scrollbar.position = int(new_value)
+            self.vertical_scrollbar.position = round(new_value)
-            self.refresh(layout=True, repaint=False)
+            if round(old_value) != round(new_value):
+                self._refresh_scroll()
 
     def validate_scroll_x(self, value: float) -> float:
         return clamp(value, 0, self.max_scroll_x)
@@ -1147,7 +1149,7 @@ class Widget(DOMNode):
         Returns:
             Offset: Offset a container has been scrolled by.
         """
-        return Offset(int(self.scroll_x), int(self.scroll_y))
+        return Offset(round(self.scroll_x), round(self.scroll_y))
 
     @property
     def is_transparent(self) -> bool:
@@ -2155,8 +2157,16 @@ class Widget(DOMNode):
         event._set_forwarded()
         await self.post_message(event)
 
+    def _refresh_scroll(self) -> None:
+        """Refreshes the scroll position."""
+        self._layout_required = True
+        self.check_idle()
+
     def refresh(
-        self, *regions: Region, repaint: bool = True, layout: bool = False
+        self,
+        *regions: Region,
+        repaint: bool = True,
+        layout: bool = False,
     ) -> None:
         """Initiate a refresh of the widget.
 

tests/test_concurrency.py

@@ -0,0 +1,53 @@
+import pytest
+
+from threading import Thread
+from textual.app import App, ComposeResult
+from textual.widgets import TextLog
+
+
+def test_call_from_thread_app_not_running():
+    app = App()
+
+    # Should fail if app is not running
+    with pytest.raises(RuntimeError):
+        app.call_from_thread(print)
+
+
+def test_call_from_thread():
+    """Test the call_from_thread method."""
+
+    class BackgroundThread(Thread):
+        """A background thread which will modify app in some way."""
+
+        def __init__(self, app: App) -> None:
+            self.app = app
+            super().__init__()
+
+        def run(self) -> None:
+            def write_stuff(text: str) -> None:
+                """Write stuff to a widget."""
+                self.app.query_one(TextLog).write(text)
+
+            self.app.call_from_thread(write_stuff, "Hello")
+            # Exit the app with a code we can assert
+            self.app.call_from_thread(self.app.exit, 123)
+
+    class ThreadTestApp(App):
+        """Trivial app with a single widget."""
+
+        def compose(self) -> ComposeResult:
+            yield TextLog()
+
+        def on_ready(self) -> None:
+            """Launch a thread which will modify the app."""
+            try:
+                self.call_from_thread(print)
+            except RuntimeError as error:
+                # Calling this from the same thread as the app is an error
+                self._runtime_error = error
+            BackgroundThread(self).start()
+
+    app = ThreadTestApp()
+    result = app.run(headless=True, size=(80, 24))
+    assert isinstance(app._runtime_error, RuntimeError)
+    assert result == 123

tests/test_immutable_sequence_view.py

@@ -24,7 +24,7 @@ def test_non_empty_immutable_sequence() -> None:
 
 def test_no_assign_to_immutable_sequence() -> None:
     """It should not be possible to assign into an immutable sequence."""
-    tester = wrap([1,2,3,4,5])
+    tester = wrap([1, 2, 3, 4, 5])
     with pytest.raises(TypeError):
         tester[0] = 23
     with pytest.raises(TypeError):
@@ -33,7 +33,7 @@ def test_no_assign_to_immutable_sequence() -> None:
 
 def test_no_del_from_iummutable_sequence() -> None:
     """It should not be possible delete an item from an immutable sequence."""
-    tester = wrap([1,2,3,4,5])
+    tester = wrap([1, 2, 3, 4, 5])
     with pytest.raises(TypeError):
         del tester[0]
 
@@ -46,23 +46,23 @@ def test_get_item_from_immutable_sequence() -> None:
 
 def test_get_slice_from_immutable_sequence() -> None:
     """It should be possible to get a slice from an immutable sequence."""
-    assert list(wrap(range(10))[0:2]) == [0,1]
+    assert list(wrap(range(10))[0:2]) == [0, 1]
-    assert list(wrap(range(10))[0:-1]) == [0,1,2,3,4,5,6,7,8]
+    assert list(wrap(range(10))[0:-1]) == [0, 1, 2, 3, 4, 5, 6, 7, 8]
 
 
 def test_immutable_sequence_contains() -> None:
     """It should be possible to see if an immutable sequence contains a value."""
-    tester = wrap([1,2,3,4,5])
+    tester = wrap([1, 2, 3, 4, 5])
     assert 1 in tester
     assert 11 not in tester
 
 
 def test_immutable_sequence_index() -> None:
-    tester = wrap([1,2,3,4,5])
+    tester = wrap([1, 2, 3, 4, 5])
     assert tester.index(1) == 0
     with pytest.raises(ValueError):
         _ = tester.index(11)
 
 
 def test_reverse_immutable_sequence() -> None:
-    assert list(reversed(wrap([1,2]))) == [2,1]
+    assert list(reversed(wrap([1, 2]))) == [2, 1]

tests/test_strip.py

@@ -1,6 +1,8 @@
+import pytest
 from rich.segment import Segment
 from rich.style import Style
 
+from textual._segment_tools import NoCellPositionForIndex
 from textual.strip import Strip
 from textual._filter import Monochrome
 
@@ -62,9 +64,7 @@ def test_eq():
 
 
 def test_adjust_cell_length():
-
     for repeat in range(3):
-
         assert Strip([]).adjust_cell_length(3) == Strip([Segment("   ")])
         assert Strip([Segment("f")]).adjust_cell_length(3) == Strip(
             [Segment("f"), Segment("  ")]
@@ -119,9 +119,7 @@ def test_style_links():
 
 
 def test_crop():
-
     for repeat in range(3):
-
         assert Strip([Segment("foo")]).crop(0, 3) == Strip([Segment("foo")])
         assert Strip([Segment("foo")]).crop(0, 2) == Strip([Segment("fo")])
         assert Strip([Segment("foo")]).crop(0, 1) == Strip([Segment("f")])
@@ -136,10 +134,42 @@ def test_crop():
 
 
 def test_divide():
-
     for repeat in range(3):
-
         assert Strip([Segment("foo")]).divide([1, 2]) == [
             Strip([Segment("f")]),
             Strip([Segment("o")]),
         ]
+
+
+@pytest.mark.parametrize(
+    "index,cell_position",
+    [
+        (0, 0),
+        (1, 1),
+        (2, 2),
+        (3, 3),
+        (4, 4),
+        (5, 6),
+        (6, 8),
+        (7, 10),
+        (8, 11),
+        (9, 12),
+        (10, 13),
+        (11, 14),
+    ],
+)
+def test_index_to_cell_position(index, cell_position):
+    strip = Strip([Segment("ab"), Segment("cd日本語ef"), Segment("gh")])
+    assert cell_position == strip.index_to_cell_position(index)
+
+
+def test_index_cell_position_no_segments():
+    strip = Strip([])
+    with pytest.raises(NoCellPositionForIndex):
+        strip.index_to_cell_position(2)
+
+
+def test_index_cell_position_index_too_large():
+    strip = Strip([Segment("abcdef"), Segment("ghi")])
+    with pytest.raises(NoCellPositionForIndex):
+        strip.index_to_cell_position(100)

tests/tree/test_tree_node_children.py

@@ -1,29 +1,29 @@
 import pytest
 from textual.widgets import Tree, TreeNode
 
-def label_of(node: TreeNode[None]):
-    """Get the label of a node.
 
-    TODO: This is just a helper function to reduce the number of type
+def label_of(node: TreeNode[None]):
-    errors, which can and will be remove once this code is merged with a
+    """Get the label of a node as a string"""
-    version of main that also has the TreeNode.label PR merged.
+    return str(node.label)
-    """
-    return str(node._label)
 
 
 def test_tree_node_children() -> None:
     """A node's children property should act like an immutable list."""
-    CHILDREN=23
+    CHILDREN = 23
     tree = Tree[None]("Root")
     for child in range(CHILDREN):
         tree.root.add(str(child))
-    assert len(tree.root.children)==CHILDREN
+    assert len(tree.root.children) == CHILDREN
     for child in range(CHILDREN):
         assert label_of(tree.root.children[child]) == str(child)
     assert label_of(tree.root.children[0]) == "0"
-    assert label_of(tree.root.children[-1]) == str(CHILDREN-1)
+    assert label_of(tree.root.children[-1]) == str(CHILDREN - 1)
-    assert [label_of(node) for node in tree.root.children] == [str(n) for n in range(CHILDREN)]
+    assert [label_of(node) for node in tree.root.children] == [
-    assert [label_of(node) for node in tree.root.children[:2]] == [str(n) for n in range(2)]
+        str(n) for n in range(CHILDREN)
+    ]
+    assert [label_of(node) for node in tree.root.children[:2]] == [
+        str(n) for n in range(2)
+    ]
     with pytest.raises(TypeError):
         tree.root.children[0] = tree.root.children[1]
     with pytest.raises(TypeError):

tests/tree/test_tree_node_label.py

@@ -1,6 +1,7 @@
 from textual.widgets import Tree, TreeNode
 from rich.text import Text
 
+
 def test_tree_node_label() -> None:
     """It should be possible to modify a TreeNode's label."""
     node = TreeNode(Tree[None]("Xenomorph Lifecycle"), None, 0, "Facehugger")
@@ -8,6 +9,7 @@ def test_tree_node_label() -> None:
     node.label = "Chestbuster"
     assert node.label == Text("Chestbuster")
 
+
 def test_tree_node_label_via_tree() -> None:
     """It should be possible to modify a TreeNode's label when created via a Tree."""
     tree = Tree[None]("Xenomorph Lifecycle")

tests/tree/test_tree_node_parent.py

@@ -1,5 +1,6 @@
 from textual.widgets import TreeNode, Tree
 
+
 def test_tree_node_parent() -> None:
     """It should be possible to access a TreeNode's parent."""
     tree = Tree[None]("Anakin")