Allow scrollbar size of zero (scrollable containers without scrollbars) (#3488)

* Remove the guard code which protects against `scrollbar-size-*:0`, update error messages indicating its supported now.

* Fix scrollbar region calculation to support zero-thickness scrollbars

* Add test ensuring zero-width scrollbars dont blow up

* Updating CHANGELOG regarding zero-thickness scrollbars

* Add note to scrollbar_size.md about zero width scrollbar

Author: darrenburns

CHANGELOG.md

@@ -28,6 +28,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Added `Input.clear` method https://github.com/Textualize/textual/pull/3430
 - Added `TextArea.SelectionChanged` and `TextArea.Changed` messages https://github.com/Textualize/textual/pull/3442
 - Added `wait_for_dismiss` parameter to `App.push_screen` https://github.com/Textualize/textual/pull/3477
+- Allow scrollbar-size to be set to 0 to achieve scrollable containers with no visible scrollbars https://github.com/Textualize/textual/pull/3488
 
 ### Changed
 

docs/styles/scrollbar_size.md

@@ -44,6 +44,11 @@ In this example we modify the size of the widget's scrollbar to be _much_ larger
 
 In the next example we show three containers with differently sized scrollbars.
 
+!!! tip
+
+    If you want to hide the scrollbar but still allow the container to scroll
+    using the mousewheel or keyboard, you can set the scrollbar size to `0`.
+
 === "Output"
 
     ```{.textual path="docs/examples/styles/scrollbar_size2.py"}

src/textual/css/_help_text.py

@@ -587,9 +587,7 @@ def scrollbar_size_property_help_text(context: StylingContext) -> HelpText:
                     ),
                 ],
             ).get_by_context(context),
-            Bullet(
-                "<horizontal> and <vertical> must be positive integers, greater than zero"
-            ),
+            Bullet("<horizontal> and <vertical> must be non-negative integers."),
         ],
     )
 

src/textual/css/_styles_builder.py

@@ -876,11 +876,7 @@ def scrollbar_size_error(name: str, token: Token) -> None:
                 scrollbar_size_error(name, token2)
 
             horizontal = int(token1.value)
-            if horizontal == 0:
-                scrollbar_size_error(name, token1)
             vertical = int(token2.value)
-            if vertical == 0:
-                scrollbar_size_error(name, token2)
             self.styles._rules["scrollbar_size_horizontal"] = horizontal
             self.styles._rules["scrollbar_size_vertical"] = vertical
             self._distribute_importance("scrollbar_size", ("horizontal", "vertical"))
@@ -895,8 +891,6 @@ def process_scrollbar_size_vertical(self, name: str, tokens: list[Token]) -> Non
             if token.name != "number" or not token.value.isdigit():
                 self.error(name, token, scrollbar_size_single_axis_help_text(name))
             value = int(token.value)
-            if value == 0:
-                self.error(name, token, scrollbar_size_single_axis_help_text(name))
             self.styles._rules["scrollbar_size_vertical"] = value
 
     def process_scrollbar_size_horizontal(self, name: str, tokens: list[Token]) -> None:
@@ -909,8 +903,6 @@ def process_scrollbar_size_horizontal(self, name: str, tokens: list[Token]) -> N
             if token.name != "number" or not token.value.isdigit():
                 self.error(name, token, scrollbar_size_single_axis_help_text(name))
             value = int(token.value)
-            if value == 0:
-                self.error(name, token, scrollbar_size_single_axis_help_text(name))
             self.styles._rules["scrollbar_size_horizontal"] = value
 
     def _process_grid_rows_or_columns(self, name: str, tokens: list[Token]) -> None:

src/textual/widget.py

@@ -1220,7 +1220,7 @@ def vertical_scrollbar(self) -> ScrollBar:
 
     @property
     def horizontal_scrollbar(self) -> ScrollBar:
-        """The a horizontal scrollbar.
+        """The horizontal scrollbar.
 
         Note:
             This will *create* a scrollbar if one doesn't exist.
@@ -2701,8 +2701,8 @@ def _arrange_scrollbars(self, region: Region) -> Iterable[tuple[Widget, Region]]
                 horizontal_scrollbar_region,
                 scrollbar_corner_gap,
             ) = region.split(
-                -scrollbar_size_vertical,
-                -scrollbar_size_horizontal,
+                region.width - scrollbar_size_vertical,
+                region.height - scrollbar_size_horizontal,
             )
             if scrollbar_corner_gap:
                 yield self.scrollbar_corner, scrollbar_corner_gap
@@ -2719,7 +2719,7 @@ def _arrange_scrollbars(self, region: Region) -> Iterable[tuple[Widget, Region]]
 
         elif show_vertical_scrollbar:
             window_region, scrollbar_region = region.split_vertical(
-                -scrollbar_size_vertical
+                region.width - scrollbar_size_vertical
             )
             if scrollbar_region:
                 scrollbar = self.vertical_scrollbar
@@ -2728,7 +2728,7 @@ def _arrange_scrollbars(self, region: Region) -> Iterable[tuple[Widget, Region]]
                 yield scrollbar, scrollbar_region
         elif show_horizontal_scrollbar:
             window_region, scrollbar_region = region.split_horizontal(
-                -scrollbar_size_horizontal
+                region.height - scrollbar_size_horizontal
             )
             if scrollbar_region:
                 scrollbar = self.horizontal_scrollbar

tests/test_containers.py

@@ -98,3 +98,24 @@ def compose(self) -> ComposeResult:
         middle = app.query_one(Middle)
         assert middle.size.width == 4
         assert middle.size.height == app.size.height
+
+
+async def test_scrollbar_zero_thickness():
+    """Ensuring that scrollbars can be set to zero thickness."""
+
+    class ScrollbarZero(App):
+        CSS = """* {
+            scrollbar-size: 0 0;
+            scrollbar-size-vertical: 0;  /* just exercising the parser */
+            scrollbar-size-horizontal: 0;  /* exercise the parser */
+        }
+        """
+
+        def compose(self) -> ComposeResult:
+            with Vertical():
+                for _ in range(10):
+                    yield Label("Hello, world!")
+
+    app = ScrollbarZero()
+    async with app.run_test(size=(8, 6)):
+        pass