Merge pull request #1823 from Textualize/optimize-scroll

Optimize scroll with a Spatial Map

CHANGELOG.md

@@ -5,7 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
-## Unreleased
+
+## [0.12.0] - Unreleased
+
+### Changed
+
+- Scrolling by page now adds to current position.
+
+### Removed
+
+- Removed `screen.visible_widgets` and `screen.widgets`
 
 ### Added
 

docs/examples/styles/height_comparison.css

@@ -28,12 +28,12 @@
 
 Screen {
     layers: ruler;
+    overflow: hidden;
 }
 
 Ruler {
     layer: ruler;
     dock: right;
-    overflow: hidden;
     width: 1;
     background: $accent;
 }

examples/dictionary.py

@@ -11,7 +11,7 @@ from rich.markdown import Markdown
 
 from textual.app import App, ComposeResult
 from textual.containers import Content
-from textual.widgets import Static, Input
+from textual.widgets import Input, Static
 
 
 class DictionaryApp(App):
@@ -41,7 +41,12 @@ class DictionaryApp(App):
         """Looks up a word."""
         url = f"https://api.dictionaryapi.dev/api/v2/entries/en/{word}"
         async with httpx.AsyncClient() as client:
-            results = (await client.get(url)).json()
+            response = await client.get(url)
+            try:
+                results = response.json()
+            except Exception:
+                self.query_one("#results", Static).update(response.text)
+                return
 
         if word == self.query_one(Input).value:
             markdown = self.make_word_markdown(results)

src/textual/_arrange.py

@@ -128,4 +128,4 @@ def arrange(
 
         placements.extend(layout_placements)
 
-    return placements, arrange_widgets, scroll_spacing
+    return DockArrangeResult(placements, arrange_widgets, scroll_spacing)

src/textual/_compositor.py

@@ -167,6 +167,7 @@ class Compositor:
     def __init__(self) -> None:
         # A mapping of Widget on to its "render location" (absolute position / depth)
         self.map: CompositorMap = {}
+        self._full_map: CompositorMap | None = None
         self._layers: list[tuple[Widget, MapGeometry]] | None = None
 
         # All widgets considered in the arrangement
@@ -241,29 +242,27 @@ class Compositor:
             size: Size of the area to be filled.
 
         Returns:
-            Hidden shown and resized widgets.
+            Hidden, shown, and resized widgets.
         """
         self._cuts = None
         self._layers = None
         self._layers_visible = None
         self._visible_widgets = None
+        self._full_map = None
         self.root = parent
         self.size = size
 
         # Keep a copy of the old map because we're going to compare it with the update
-        old_map = self.map.copy()
+        old_map = self.map
         old_widgets = old_map.keys()
 
         map, widgets = self._arrange_root(parent, size)
-        new_widgets = map.keys()
 
-        # Newly visible widgets
+        new_widgets = map.keys()
-        shown_widgets = new_widgets - old_widgets
-        # Newly hidden widgets
-        hidden_widgets = old_widgets - new_widgets
 
         # Replace map and widgets
         self.map = map
+        self._full_map = map
         self.widgets = widgets
 
         # Contains widgets + geometry for every widget that changed (added, removed, or updated)
@@ -272,13 +271,7 @@ class Compositor:
         # Widgets in both new and old
         common_widgets = old_widgets & new_widgets
 
-        # Widgets with changed size
+        # Mark dirty regions.
-        resized_widgets = {
-            widget
-            for widget, (region, *_) in changes
-            if (widget in common_widgets and old_map[widget].region[2:] != region[2:])
-        }
-
         screen_region = size.region
         if screen_region not in self._dirty_regions:
             regions = {
@@ -291,12 +284,80 @@ class Compositor:
             }
             self._dirty_regions.update(regions)
 
+        resized_widgets = {
+            widget
+            for widget, (region, *_) in changes
+            if (widget in common_widgets and old_map[widget].region[2:] != region[2:])
+        }
+        # Newly visible widgets
+        shown_widgets = new_widgets - old_widgets
+        # Newly hidden widgets
+        hidden_widgets = self.widgets - widgets
         return ReflowResult(
             hidden=hidden_widgets,
             shown=shown_widgets,
             resized=resized_widgets,
         )
 
+    def reflow_visible(self, parent: Widget, size: Size) -> set[Widget]:
+        """Reflow only the visible children.
+
+        This is a fast-path for scrolling.
+
+        Args:
+            parent: The root widget.
+            size: Size of the area to be filled.
+
+        Returns:
+            Set of widgets that were exposed by the scroll.
+
+        """
+        self._cuts = None
+        self._layers = None
+        self._layers_visible = None
+        self._visible_widgets = None
+        self._full_map = None
+        self.root = parent
+        self.size = size
+
+        # Keep a copy of the old map because we're going to compare it with the update
+        old_map = self.map
+        map, widgets = self._arrange_root(parent, size, visible_only=True)
+
+        exposed_widgets = map.keys() - old_map.keys()
+        # Replace map and widgets
+        self.map = map
+        self.widgets = widgets
+
+        # Contains widgets + geometry for every widget that changed (added, removed, or updated)
+        changes = map.items() ^ old_map.items()
+
+        # Mark dirty regions.
+        screen_region = size.region
+        if screen_region not in self._dirty_regions:
+            regions = {
+                region
+                for region in (
+                    map_geometry.clip.intersection(map_geometry.region)
+                    for _, map_geometry in changes
+                )
+                if region
+            }
+            self._dirty_regions.update(regions)
+
+        return exposed_widgets
+
+    @property
+    def full_map(self) -> CompositorMap:
+        """Lazily built compositor map that covers all widgets."""
+        if self.root is None or not self.map:
+            return {}
+        if self._full_map is None:
+            map, widgets = self._arrange_root(self.root, self.size, visible_only=False)
+            self._full_map = map
+
+        return self._full_map
+
     @property
     def visible_widgets(self) -> dict[Widget, tuple[Region, Region]]:
         """Get a mapping of widgets on to region and clip.
@@ -322,9 +383,9 @@ class Compositor:
         return self._visible_widgets
 
     def _arrange_root(
-        self, root: Widget, size: Size
+        self, root: Widget, size: Size, visible_only: bool = True
     ) -> tuple[CompositorMap, set[Widget]]:
-        """Arrange a widgets children based on its layout attribute.
+        """Arrange a widget's children based on its layout attribute.
 
         Args:
             root: Top level widget.
@@ -337,6 +398,7 @@ class Compositor:
 
         map: CompositorMap = {}
         widgets: set[Widget] = set()
+        add_new_widget = widgets.add
         layer_order: int = 0
 
         def add_widget(
@@ -362,7 +424,7 @@ class Compositor:
                 visible = visibility == "visible"
 
             if visible:
-                widgets.add(widget)
+                add_new_widget(widget)
             styles_offset = widget.styles.offset
             layout_offset = (
                 styles_offset.resolve(region.size, clip.size)
@@ -389,22 +451,26 @@ class Compositor:
 
                 if widget.is_container:
                     # Arrange the layout
-                    placements, arranged_widgets, spacing = widget._arrange(
+                    arrange_result = widget._arrange(child_region.size)
-                        child_region.size
+                    arranged_widgets = arrange_result.widgets
-                    )
+                    spacing = arrange_result.spacing
                     widgets.update(arranged_widgets)
 
-                    if placements:
+                    if visible_only:
+                        placements = arrange_result.get_visible_placements(
+                            container_size.region + widget.scroll_offset
+                        )
+                    else:
+                        placements = arrange_result.placements
+                    total_region = total_region.union(arrange_result.total_region)
+
                     # An offset added to all placements
                     placement_offset = container_region.offset
-                        placement_scroll_offset = (
+                    placement_scroll_offset = placement_offset - widget.scroll_offset
-                            placement_offset - widget.scroll_offset
-                        )
 
                     _layers = widget.layers
                     layers_to_index = {
-                            layer_name: index
+                        layer_name: index for index, layer_name in enumerate(_layers)
-                            for index, layer_name in enumerate(_layers)
                     }
                     get_layer_index = layers_to_index.get
 
@@ -437,10 +503,12 @@ class Compositor:
                             sub_clip,
                             visible,
                         )
+
                         layer_order -= 1
 
                 if visible:
                     # Add any scrollbars
+                    if any(widget.scrollbars_enabled):
                         for chrome_widget, chrome_region in widget._arrange_scrollbars(
                             container_region
                         ):
@@ -518,6 +586,9 @@ class Compositor:
         """Get the offset of a widget."""
         try:
             return self.map[widget].region.offset
+        except KeyError:
+            try:
+                return self.full_map[widget].region.offset
             except KeyError:
                 raise errors.NoWidget("Widget is not in layout")
 
@@ -601,8 +672,13 @@ class Compositor:
             Widget's composition information.
 
         """
+        if self.root is None or not self.map:
+            raise errors.NoWidget("Widget is not in layout")
         try:
             region = self.map[widget]
+        except KeyError:
+            try:
+                return self.full_map[widget]
             except KeyError:
                 raise errors.NoWidget("Widget is not in layout")
         else:
@@ -788,6 +864,7 @@ class Compositor:
             widget: Widget to update.
 
         """
+        self._full_map = None
         regions: list[Region] = []
         add_region = regions.append
         get_widget = self.visible_widgets.__getitem__

src/textual/_layout.py

@@ -1,8 +1,10 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
+from dataclasses import dataclass
 from typing import TYPE_CHECKING, ClassVar, NamedTuple
 
+from ._spatial_map import SpatialMap
 from .geometry import Region, Size, Spacing
 
 if TYPE_CHECKING:
@@ -11,7 +13,55 @@ if TYPE_CHECKING:
     from .widget import Widget
 
 ArrangeResult: TypeAlias = "tuple[list[WidgetPlacement], set[Widget]]"
-DockArrangeResult: TypeAlias = "tuple[list[WidgetPlacement], set[Widget], Spacing]"
+
+
+@dataclass
+class DockArrangeResult:
+    placements: list[WidgetPlacement]
+    """A `WidgetPlacement` for every widget to describe it's location on screen."""
+    widgets: set[Widget]
+    """A set of widgets in the arrangement."""
+    spacing: Spacing
+    """Shared spacing around the widgets."""
+
+    _spatial_map: SpatialMap[WidgetPlacement] | None = None
+
+    @property
+    def spatial_map(self) -> SpatialMap[WidgetPlacement]:
+        """A lazy-calculated spatial map."""
+        if self._spatial_map is None:
+            self._spatial_map = SpatialMap()
+            self._spatial_map.insert(
+                (
+                    placement.region.grow(placement.margin),
+                    placement.fixed,
+                    placement,
+                )
+                for placement in self.placements
+            )
+
+        return self._spatial_map
+
+    @property
+    def total_region(self) -> Region:
+        """The total area occupied by the arrangement.
+
+        Returns:
+            A Region.
+        """
+        return self.spatial_map.total_region
+
+    def get_visible_placements(self, region: Region) -> list[WidgetPlacement]:
+        """Get the placements visible within the given region.
+
+        Args:
+            region: A region.
+
+        Returns:
+            Set of placements.
+        """
+        visible_placements = self.spatial_map.get_values_in_region(region)
+        return visible_placements
 
 
 class WidgetPlacement(NamedTuple):
@@ -61,7 +111,7 @@ class Layout(ABC):
             width = 0
         else:
             # Use a size of 0, 0 to ignore relative sizes, since those are flexible anyway
-            placements, _, _ = widget._arrange(Size(0, 0))
+            placements = widget._arrange(Size(0, 0)).placements
             width = max(
                 [
                     placement.region.right + placement.margin.right
@@ -89,7 +139,7 @@ class Layout(ABC):
             height = 0
         else:
             # Use a height of zero to ignore relative heights
-            placements, _, _ = widget._arrange(Size(width, 0))
+            placements = widget._arrange(Size(width, 0)).placements
             height = max(
                 [
                     placement.region.bottom + placement.margin.bottom

src/textual/_spatial_map.py

@@ -0,0 +1,103 @@
+from __future__ import annotations
+
+from collections import defaultdict
+from itertools import product
+from typing import Generic, Iterable, TypeVar
+
+from typing_extensions import TypeAlias
+
+from .geometry import Region
+
+ValueType = TypeVar("ValueType")
+GridCoordinate: TypeAlias = "tuple[int, int]"
+
+
+class SpatialMap(Generic[ValueType]):
+    """A spatial map allows for data to be associated with rectangular regions
+    in Euclidean space, and efficiently queried.
+
+    When the SpatialMap is populated, a reference to each value is placed into one or
+    more buckets associated with a regular grid that covers 2D space.
+
+    The SpatialMap is able to quickly retrieve the values under a given "window" region
+    by combining the values in the grid squares under the visible area.
+
+    """
+
+    def __init__(self, grid_width: int = 100, grid_height: int = 20) -> None:
+        """Create a spatial map with the given grid size.
+
+        Args:
+            grid_width: Width of a grid square.
+            grid_height: Height of a grid square.
+        """
+        self._grid_size = (grid_width, grid_height)
+        self.total_region = Region()
+        self._map: defaultdict[GridCoordinate, list[ValueType]] = defaultdict(list)
+        self._fixed: list[ValueType] = []
+
+    def _region_to_grid_coordinates(self, region: Region) -> Iterable[GridCoordinate]:
+        """Get the grid squares under a region.
+
+        Args:
+            region: A region.
+
+        Returns:
+            Iterable of grid coordinates (tuple of 2 values).
+        """
+        # (x1, y1) is the coordinate of the top left cell
+        # (x2, y2) is the coordinate of the bottom right cell
+        x1, y1, width, height = region
+        x2 = x1 + width - 1
+        y2 = y1 + height - 1
+        grid_width, grid_height = self._grid_size
+
+        return product(
+            range(x1 // grid_width, x2 // grid_width + 1),
+            range(y1 // grid_height, y2 // grid_height + 1),
+        )
+
+    def insert(
+        self, regions_and_values: Iterable[tuple[Region, bool, ValueType]]
+    ) -> None:
+        """Insert values into the Spatial map.
+
+        Values are associated with their region in Euclidean space, and a boolean that
+        indicates fixed regions. Fixed regions don't scroll and are always visible.
+
+        Args:
+            regions_and_values: An iterable of (REGION, FIXED, VALUE).
+        """
+        append_fixed = self._fixed.append
+        get_grid_list = self._map.__getitem__
+        _region_to_grid = self._region_to_grid_coordinates
+        total_region = self.total_region
+        for region, fixed, value in regions_and_values:
+            total_region = total_region.union(region)
+            if fixed:
+                append_fixed(value)
+            else:
+                for grid in _region_to_grid(region):
+                    get_grid_list(grid).append(value)
+        self.total_region = total_region
+
+    def get_values_in_region(self, region: Region) -> list[ValueType]:
+        """Get a superset of all the values that intersect with a given region.
+
+        Note that this may return false positives.
+
+        Args:
+            region: A region.
+
+        Returns:
+            Values under the region.
+        """
+        results: list[ValueType] = self._fixed.copy()
+        add_results = results.extend
+        get_grid_values = self._map.get
+        for grid_coordinate in self._region_to_grid_coordinates(region):
+            grid_values = get_grid_values(grid_coordinate)
+            if grid_values is not None:
+                add_results(grid_values)
+        unique_values = list(dict.fromkeys(results))
+        return unique_values

src/textual/app.py

@@ -1783,6 +1783,7 @@ class App(Generic[ReturnType], DOMNode):
             await child._close_messages()
 
     async def _shutdown(self) -> None:
+        self._begin_update()  # Prevents any layout / repaint while shutting down
         driver = self._driver
         self._running = False
         if driver is not None:
@@ -1908,7 +1909,6 @@ class App(Generic[ReturnType], DOMNode):
         # Handle input events that haven't been forwarded
         # If the event has been forwarded it may have bubbled up back to the App
         if isinstance(event, events.Compose):
-            self.log(event)
             screen = Screen(id="_default")
             self._register(self, screen)
             self._screen_stack.append(screen)

src/textual/messages.py

@@ -45,12 +45,24 @@ class Update(Message, verbose=True):
 
 @rich.repr.auto
 class Layout(Message, verbose=True):
+    """Sent by Textual when a layout is required."""
+
     def can_replace(self, message: Message) -> bool:
         return isinstance(message, Layout)
 
 
+@rich.repr.auto
+class UpdateScroll(Message, verbose=True):
+    """Sent by Textual when a scroll update is required."""
+
+    def can_replace(self, message: Message) -> bool:
+        return isinstance(message, UpdateScroll)
+
+
 @rich.repr.auto
 class InvokeLater(Message, verbose=True, bubble=False):
+    """Sent by Textual to invoke a callback."""
+
     def __init__(self, sender: MessagePump, callback: CallbackType) -> None:
         self.callback = callback
         super().__init__(sender)

src/textual/screen.py

@@ -80,16 +80,6 @@ class Screen(Widget):
             )
         return self._update_timer
 
-    @property
-    def widgets(self) -> list[Widget]:
-        """Get all widgets."""
-        return list(self._compositor.map.keys())
-
-    @property
-    def visible_widgets(self) -> list[Widget]:
-        """Get a list of visible widgets."""
-        return list(self._compositor.visible_widgets)
-
     def render(self) -> RenderableType:
         background = self.styles.background
         if background.is_transparent:
@@ -370,7 +360,12 @@ class Screen(Widget):
                     if self._layout_required:
                         self._refresh_layout()
                         self._layout_required = False
+                        self._scroll_required = False
                         self._dirty_widgets.clear()
+                    elif self._scroll_required:
+                        self._refresh_layout(scroll=True)
+                        self._scroll_required = False
+
                     if self._repaint_required:
                         self._dirty_widgets.clear()
                         self._dirty_widgets.add(self)
@@ -419,7 +414,9 @@ class Screen(Widget):
         self._callbacks.append(callback)
         self.check_idle()
 
-    def _refresh_layout(self, size: Size | None = None, full: bool = False) -> None:
+    def _refresh_layout(
+        self, size: Size | None = None, full: bool = False, scroll: bool = False
+    ) -> None:
         """Refresh the layout (can change size and positions of widgets)."""
         size = self.outer_size if size is None else size
         if not size:
@@ -427,7 +424,37 @@ class Screen(Widget):
 
         self._compositor.update_widgets(self._dirty_widgets)
         self.update_timer.pause()
+        ResizeEvent = events.Resize
         try:
+            if scroll:
+                exposed_widgets = self._compositor.reflow_visible(self, size)
+                if exposed_widgets:
+                    layers = self._compositor.layers
+
+                    for widget, (
+                        region,
+                        _order,
+                        _clip,
+                        virtual_size,
+                        container_size,
+                        _,
+                    ) in layers:
+                        if widget in exposed_widgets:
+                            if widget._size_updated(
+                                region.size,
+                                virtual_size,
+                                container_size,
+                                layout=False,
+                            ):
+                                widget.post_message_no_wait(
+                                    ResizeEvent(
+                                        self,
+                                        region.size,
+                                        virtual_size,
+                                        container_size,
+                                    )
+                                )
+            else:
                 hidden, shown, resized = self._compositor.reflow(self, size)
                 Hide = events.Hide
                 Show = events.Show
@@ -437,7 +464,6 @@ class Screen(Widget):
 
                 # We want to send a resize event to widgets that were just added or change since last layout
                 send_resize = shown | resized
-            ResizeEvent = events.Resize
 
                 layers = self._compositor.layers
                 for widget, (
@@ -480,6 +506,12 @@ class Screen(Widget):
         self._layout_required = True
         self.check_idle()
 
+    async def _on_update_scroll(self, message: messages.UpdateScroll) -> None:
+        message.stop()
+        message.prevent_default()
+        self._scroll_required = True
+        self.check_idle()
+
     def _screen_resized(self, size: Size):
         """Called by App when the screen is resized."""
         self._refresh_layout(size, full=True)

src/textual/scroll_view.py

@@ -69,14 +69,18 @@ class ScrollView(Widget):
         return self.virtual_size.height
 
     def _size_updated(
-        self, size: Size, virtual_size: Size, container_size: Size
+        self, size: Size, virtual_size: Size, container_size: Size, layout: bool = True
-    ) -> None:
+    ) -> bool:
         """Called when size is updated.
 
         Args:
             size: New size.
             virtual_size: New virtual size.
             container_size: New container size.
+            layout: Perform layout if required.
+
+        Returns:
+            True if anything changed, or False if nothing changed.
         """
         if self._size != size or container_size != container_size:
             self.refresh()
@@ -90,6 +94,9 @@ class ScrollView(Widget):
             self._container_size = size - self.styles.gutter.totals
             self._scroll_update(virtual_size)
             self.scroll_to(self.scroll_x, self.scroll_y, animate=False)
+            return True
+        else:
+            return False
 
     def render(self) -> RenderableType:
         """Render the scrollable region (if `render_lines` is not implemented).

src/textual/widget.py

@@ -38,6 +38,7 @@ from . import errors, events, messages
 from ._animator import DEFAULT_EASING, Animatable, BoundAnimator, EasingFunction
 from ._arrange import DockArrangeResult, arrange
 from ._asyncio import create_task
+from ._cache import FIFOCache
 from ._context import active_app
 from ._easing import DEFAULT_SCROLL_EASING
 from ._layout import Layout
@@ -243,6 +244,7 @@ class Widget(DOMNode):
         self._container_size = Size(0, 0)
         self._layout_required = False
         self._repaint_required = False
+        self._scroll_required = False
         self._default_layout = VerticalLayout()
         self._animate: BoundAnimator | None = None
         self.highlight_style: Style | None = None
@@ -262,8 +264,9 @@ class Widget(DOMNode):
         self._content_width_cache: tuple[object, int] = (None, 0)
         self._content_height_cache: tuple[object, int] = (None, 0)
 
-        self._arrangement_cache_key: tuple[Size, int] = (Size(), -1)
+        self._arrangement_cache: FIFOCache[
-        self._cached_arrangement: DockArrangeResult | None = None
+            tuple[Size, int], DockArrangeResult
+        ] = FIFOCache(4)
 
         self._styles_cache = StylesCache()
         self._rich_style_cache: dict[str, tuple[Style, Style]] = {}
@@ -477,14 +480,11 @@ class Widget(DOMNode):
         assert self.is_container
 
         cache_key = (size, self._nodes._updates)
-        if (
+        cached_result = self._arrangement_cache.get(cache_key)
-            self._arrangement_cache_key == cache_key
+        if cached_result is not None:
-            and self._cached_arrangement is not None
+            return cached_result
-        ):
-            return self._cached_arrangement
 
-        self._arrangement_cache_key = cache_key
+        arrangement = self._arrangement_cache[cache_key] = arrange(
-        arrangement = self._cached_arrangement = arrange(
             self, self._nodes, size, self.screen.size
         )
 
@@ -492,7 +492,7 @@ class Widget(DOMNode):
 
     def _clear_arrangement_cache(self) -> None:
         """Clear arrangement cache, forcing a new arrange operation."""
-        self._cached_arrangement = None
+        self._arrangement_cache.clear()
 
     def _get_virtual_dom(self) -> Iterable[Widget]:
         """Get widgets not part of the DOM.
@@ -1728,7 +1728,7 @@ class Widget(DOMNode):
 
         """
         return self.scroll_to(
-            y=self.scroll_target_y - self.container_size.height,
+            y=self.scroll_y - self.container_size.height,
             animate=animate,
             speed=speed,
             duration=duration,
@@ -1760,7 +1760,7 @@ class Widget(DOMNode):
 
         """
         return self.scroll_to(
-            y=self.scroll_target_y + self.container_size.height,
+            y=self.scroll_y + self.container_size.height,
             animate=animate,
             speed=speed,
             duration=duration,
@@ -1794,7 +1794,7 @@ class Widget(DOMNode):
         if speed is None and duration is None:
             duration = 0.3
         return self.scroll_to(
-            x=self.scroll_target_x - self.container_size.width,
+            x=self.scroll_x - self.container_size.width,
             animate=animate,
             speed=speed,
             duration=duration,
@@ -1828,7 +1828,7 @@ class Widget(DOMNode):
         if speed is None and duration is None:
             duration = 0.3
         return self.scroll_to(
-            x=self.scroll_target_x + self.container_size.width,
+            x=self.scroll_x + self.container_size.width,
             animate=animate,
             speed=speed,
             duration=duration,
@@ -2164,14 +2164,18 @@ class Widget(DOMNode):
         self._update_styles()
 
     def _size_updated(
-        self, size: Size, virtual_size: Size, container_size: Size
+        self, size: Size, virtual_size: Size, container_size: Size, layout: bool = True
-    ) -> None:
+    ) -> bool:
         """Called when the widget's size is updated.
 
         Args:
             size: Screen size.
             virtual_size: Virtual (scrollable) size.
             container_size: Container size (size of parent).
+            layout: Perform layout if required.
+
+        Returns:
+            True if anything changed, or False if nothing changed.
         """
         if (
             self._size != size
@@ -2179,11 +2183,16 @@ class Widget(DOMNode):
             or self._container_size != container_size
         ):
             self._size = size
+            if layout:
                 self.virtual_size = virtual_size
+            else:
+                self._reactive_virtual_size = virtual_size
             self._container_size = container_size
             if self.is_scrollable:
                 self._scroll_update(virtual_size)
-            self.refresh()
+            return True
+        else:
+            return False
 
     def _scroll_update(self, virtual_size: Size) -> None:
         """Update scrollbars visibility and dimensions.
@@ -2294,7 +2303,7 @@ class Widget(DOMNode):
 
     def _refresh_scroll(self) -> None:
         """Refreshes the scroll position."""
-        self._layout_required = True
+        self._scroll_required = True
         self.check_idle()
 
     def refresh(
@@ -2321,8 +2330,7 @@ class Widget(DOMNode):
             repaint: Repaint the widget (will call render() again). Defaults to True.
             layout: Also layout widgets in the view. Defaults to False.
         """
-
+        if layout and not self._layout_required:
-        if layout:
             self._layout_required = True
             for ancestor in self.ancestors:
                 if not isinstance(ancestor, Widget):
@@ -2403,6 +2411,9 @@ class Widget(DOMNode):
             except NoScreen:
                 pass
             else:
+                if self._scroll_required:
+                    self._scroll_required = False
+                    screen.post_message_no_wait(messages.UpdateScroll(self))
                 if self._repaint_required:
                     self._repaint_required = False
                     screen.post_message_no_wait(messages.Update(self, self))

src/textual/widgets/_placeholder.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from itertools import cycle
 
+from rich.console import RenderableType
 from typing_extensions import Literal
 
 from .. import events
@@ -61,10 +62,10 @@ class Placeholder(Widget):
         overflow: hidden;
         color: $text;
     }
-
     Placeholder.-text {
         padding: 1;
     }
+
     """
 
     # Consecutive placeholders get assigned consecutive colors.
@@ -73,7 +74,7 @@ class Placeholder(Widget):
 
     variant: Reactive[PlaceholderVariant] = reactive("default")
 
-    _renderables: dict[PlaceholderVariant, RenderResult]
+    _renderables: dict[PlaceholderVariant, str]
 
     @classmethod
     def reset_color_cycle(cls) -> None:
@@ -119,7 +120,7 @@ class Placeholder(Widget):
         while next(self._variants_cycle) != self.variant:
             pass
 
-    def render(self) -> RenderResult:
+    def render(self) -> RenderableType:
         return self._renderables[self.variant]
 
     def cycle_variant(self) -> None:
@@ -147,6 +148,6 @@ class Placeholder(Widget):
 
     def on_resize(self, event: events.Resize) -> None:
         """Update the placeholder "size" variant with the new placeholder size."""
-        self._renderables["size"] = self._SIZE_RENDER_TEMPLATE.format(*self.size)
+        self._renderables["size"] = self._SIZE_RENDER_TEMPLATE.format(*event.size)
         if self.variant == "size":
-            self.refresh(layout=True)
+            self.refresh(layout=False)

tests/test_arrange.py

@@ -9,10 +9,10 @@ from textual.widget import Widget
 def test_arrange_empty():
     container = Widget(id="container")
 
-    placements, widgets, spacing = arrange(container, [], Size(80, 24), Size(80, 24))
+    result = arrange(container, [], Size(80, 24), Size(80, 24))
-    assert placements == []
+    assert result.placements == []
-    assert widgets == set()
+    assert result.widgets == set()
-    assert spacing == Spacing(0, 0, 0, 0)
+    assert result.spacing == Spacing(0, 0, 0, 0)
 
 
 def test_arrange_dock_top():
@@ -22,17 +22,16 @@ def test_arrange_dock_top():
     header.styles.dock = "top"
     header.styles.height = "1"
 
-    placements, widgets, spacing = arrange(
+    result = arrange(container, [child, header], Size(80, 24), Size(80, 24))
-        container, [child, header], Size(80, 24), Size(80, 24)
+
-    )
+    assert result.placements == [
-    assert placements == [
         WidgetPlacement(
             Region(0, 0, 80, 1), Spacing(), header, order=TOP_Z, fixed=True
         ),
         WidgetPlacement(Region(0, 1, 80, 23), Spacing(), child, order=0, fixed=False),
     ]
-    assert widgets == {child, header}
+    assert result.widgets == {child, header}
-    assert spacing == Spacing(1, 0, 0, 0)
+    assert result.spacing == Spacing(1, 0, 0, 0)
 
 
 def test_arrange_dock_left():
@@ -42,17 +41,15 @@ def test_arrange_dock_left():
     header.styles.dock = "left"
     header.styles.width = "10"
 
-    placements, widgets, spacing = arrange(
+    result = arrange(container, [child, header], Size(80, 24), Size(80, 24))
-        container, [child, header], Size(80, 24), Size(80, 24)
+    assert result.placements == [
-    )
-    assert placements == [
         WidgetPlacement(
             Region(0, 0, 10, 24), Spacing(), header, order=TOP_Z, fixed=True
         ),
         WidgetPlacement(Region(10, 0, 70, 24), Spacing(), child, order=0, fixed=False),
     ]
-    assert widgets == {child, header}
+    assert result.widgets == {child, header}
-    assert spacing == Spacing(0, 0, 0, 10)
+    assert result.spacing == Spacing(0, 0, 0, 10)
 
 
 def test_arrange_dock_right():
@@ -62,17 +59,15 @@ def test_arrange_dock_right():
     header.styles.dock = "right"
     header.styles.width = "10"
 
-    placements, widgets, spacing = arrange(
+    result = arrange(container, [child, header], Size(80, 24), Size(80, 24))
-        container, [child, header], Size(80, 24), Size(80, 24)
+    assert result.placements == [
-    )
-    assert placements == [
         WidgetPlacement(
             Region(70, 0, 10, 24), Spacing(), header, order=TOP_Z, fixed=True
         ),
         WidgetPlacement(Region(0, 0, 70, 24), Spacing(), child, order=0, fixed=False),
     ]
-    assert widgets == {child, header}
+    assert result.widgets == {child, header}
-    assert spacing == Spacing(0, 10, 0, 0)
+    assert result.spacing == Spacing(0, 10, 0, 0)
 
 
 def test_arrange_dock_bottom():
@@ -82,17 +77,15 @@ def test_arrange_dock_bottom():
     header.styles.dock = "bottom"
     header.styles.height = "1"
 
-    placements, widgets, spacing = arrange(
+    result = arrange(container, [child, header], Size(80, 24), Size(80, 24))
-        container, [child, header], Size(80, 24), Size(80, 24)
+    assert result.placements == [
-    )
-    assert placements == [
         WidgetPlacement(
             Region(0, 23, 80, 1), Spacing(), header, order=TOP_Z, fixed=True
         ),
         WidgetPlacement(Region(0, 0, 80, 23), Spacing(), child, order=0, fixed=False),
     ]
-    assert widgets == {child, header}
+    assert result.widgets == {child, header}
-    assert spacing == Spacing(0, 0, 1, 0)
+    assert result.spacing == Spacing(0, 0, 1, 0)
 
 
 def test_arrange_dock_badly():

tests/test_spatial_map.py

@@ -0,0 +1,64 @@
+import pytest
+
+from textual._spatial_map import SpatialMap
+from textual.geometry import Region
+
+
+@pytest.mark.parametrize(
+    "region,grid",
+    [
+        (
+            Region(0, 0, 10, 10),
+            [
+                (0, 0),
+            ],
+        ),
+        (
+            Region(10, 10, 10, 10),
+            [
+                (1, 1),
+            ],
+        ),
+        (
+            Region(0, 0, 11, 11),
+            [(0, 0), (0, 1), (1, 0), (1, 1)],
+        ),
+        (
+            Region(5, 5, 15, 3),
+            [(0, 0), (1, 0)],
+        ),
+        (
+            Region(5, 5, 2, 15),
+            [(0, 0), (0, 1)],
+        ),
+    ],
+)
+def test_region_to_grid(region, grid):
+    spatial_map = SpatialMap(10, 10)
+
+    assert list(spatial_map._region_to_grid_coordinates(region)) == grid
+
+
+def test_get_values_in_region() -> None:
+    spatial_map: SpatialMap[str] = SpatialMap(20, 10)
+
+    spatial_map.insert(
+        [
+            (Region(10, 5, 5, 5), False, "foo"),
+            (Region(5, 20, 5, 5), False, "bar"),
+            (Region(0, 0, 40, 1), True, "title"),
+        ]
+    )
+
+    assert spatial_map.get_values_in_region(Region(0, 0, 10, 5)) == [
+        "title",
+        "foo",
+    ]
+    assert spatial_map.get_values_in_region(Region(0, 1, 10, 5)) == ["title", "foo"]
+    assert spatial_map.get_values_in_region(Region(0, 10, 10, 5)) == ["title"]
+    assert spatial_map.get_values_in_region(Region(0, 20, 10, 5)) == ["title", "bar"]
+    assert spatial_map.get_values_in_region(Region(5, 5, 50, 50)) == [
+        "title",
+        "foo",
+        "bar",
+    ]

tests/test_visibility_change.py

@@ -26,21 +26,18 @@ class VisibleTester(App[None]):
 async def test_visibility_changes() -> None:
     """Test changing visibility via code and CSS."""
     async with VisibleTester().run_test() as pilot:
-        assert len(pilot.app.screen.visible_widgets) == 5
         assert pilot.app.query_one("#keep").visible is True
         assert pilot.app.query_one("#hide-via-code").visible is True
         assert pilot.app.query_one("#hide-via-css").visible is True
 
         pilot.app.query_one("#hide-via-code").styles.visibility = "hidden"
         await pilot.pause(0)
-        assert len(pilot.app.screen.visible_widgets) == 4
         assert pilot.app.query_one("#keep").visible is True
         assert pilot.app.query_one("#hide-via-code").visible is False
         assert pilot.app.query_one("#hide-via-css").visible is True
 
         pilot.app.query_one("#hide-via-css").set_class(True, "hidden")
         await pilot.pause(0)
-        assert len(pilot.app.screen.visible_widgets) == 3
         assert pilot.app.query_one("#keep").visible is True
         assert pilot.app.query_one("#hide-via-code").visible is False
         assert pilot.app.query_one("#hide-via-css").visible is False