render: sub-canvases inherit the root layout viewport for fluid resolution

Content drawn inside a scroll rendered smaller than identical content outside it: the scroll viewport draws its child into a sub-canvas sized to the viewport rect, and Canvas::viewport_layout / viewport_logical — the viewports that geom_px and font_px resolve fluid Lengths against — returned the canvas's own size. Inside a 312 px sub-canvas on a 360 px surface every fluid geometry (icon sizes, row heights, paddings) and font size resolved against 312 instead of 360, shrinking the scroll content by the width ratio, ~13 % in a phone-sized window with 24 px margins. First observed in Eydos Settings' Wi-Fi list, where the connected network row sits outside the scroll and the rest inside: the same full-fan signal icon measured 21x18 px outside and 18x16 px inside.
Add a layout_viewport field to SoftwareCanvas and GlesCanvas, None on root canvases and set by sub_canvas to the parent's effective layout viewport, so nested sub-canvases keep propagating the root surface size. viewport_layout and viewport_logical consult the inherited value before falling back to the canvas size. Root canvases are untouched, and the GLES clip layers, which reuse sub_canvas, get the same correction.

CHANGELOG.md

@@ -21,6 +21,10 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 - **Renamed** `set_design_reference` / `design_reference` → `set_fluid_reference` / `fluid_reference`. **`Length::dp` changed meaning** — it used to be a surface-proportional value, and that behaviour now lives on `Length::fluid`; `dp` is the constant-physical-size unit.
 - **Widget struct fields are now `pub( crate )`** (configured through builders), except the value / state types apps read or construct (`Time`, `Date`, `ComboState`).
 
+### Fixed
+
+- **Content inside a `scroll` no longer renders smaller than the rest of the surface.** Scroll viewports draw their child into a sub-canvas sized to the viewport rect, and fluid `Length` resolution (`Canvas::geom_px` / `font_px` — icon sizes, row heights, paddings, font sizes) resolved against that smaller canvas instead of the surface, shrinking everything inside any scroll by the width ratio (~13 % in a 360 px window with 24 px margins). Sub-canvases now inherit the root canvas's layout viewport (`viewport_layout` / `viewport_logical`), propagated through nested sub-canvases, so geometry resolves identically inside and outside offscreen content.
+
 ## [0.2.0] — 2026-06-25
 
 This release adds the primitives an embedder needs to drive ltk as the render backend for a retained, externally-owned widget tree (for example projecting an Android view hierarchy onto an ltk surface). Each is kept general rather than tied to one consumer.

src/gles_render/mod.rs

@@ -151,6 +151,12 @@ pub struct GlesCanvas
 	/// falls back to [`Self::font`].
 	pub font_registry: Option<Arc<FontRegistry>>,
 	pub dpi_scale:    f32,
+	/// Layout-resolution viewport inherited from the parent canvas.
+	/// `None` on a root canvas (resolve against own size); sub-canvases
+	/// carry the root surface size so fluid geometry and fonts resolve
+	/// the same inside offscreen content (scroll viewports, clip layers)
+	/// as outside it.
+	pub( crate ) layout_viewport: Option<( f32, f32 )>,
 	pub global_alpha: f32,
 	pub width:        u32,
 	pub height:       u32,

src/gles_render/setup.rs

@@ -336,6 +336,7 @@ impl GlesCanvas
 			font_face,
 			font_registry:  None,
 			dpi_scale:      1.0,
+			layout_viewport: None,
 			global_alpha:   1.0,
 			width,
 			height,
@@ -547,6 +548,8 @@ impl GlesCanvas
 			font_face:         self.font_face,
 			font_registry:     self.font_registry.as_ref().map( Arc::clone ),
 			dpi_scale:         self.dpi_scale,
+			layout_viewport:   Some( self.layout_viewport.unwrap_or(
+				( self.width as f32, self.height as f32 ) ) ),
 			global_alpha:      self.global_alpha,
 			width,
 			height,

src/render/mod.rs

@@ -137,6 +137,12 @@ pub struct SoftwareCanvas
 	pub font_registry: Option<Arc<FontRegistry>>,
 	/// DPI scale factor applied to font sizes.
 	pub dpi_scale: f32,
+	/// Layout-resolution viewport inherited from the parent canvas.
+	/// `None` on a root canvas (resolve against own size); sub-canvases
+	/// carry the root surface size so fluid geometry and fonts resolve
+	/// the same inside offscreen content (scroll viewports, clip layers)
+	/// as outside it.
+	pub( crate ) layout_viewport: Option<( f32, f32 )>,
 	/// Global alpha multiplier for all drawing operations (0.0 =
 	/// transparent, 1.0 = opaque).
 	pub global_alpha: f32,
@@ -216,13 +222,21 @@ impl Canvas
 	/// ```
 	pub fn viewport_logical( &self ) -> ( f32, f32 )
 	{
-		let ( pw, ph ) = self.size();
+		let ( pw, ph ) = match self.layout_viewport()
+		{
+			Some( ( w, h ) ) => ( w, h ),
+			None =>
+			{
+				let ( w, h ) = self.size();
+				( w as f32, h as f32 )
+			}
+		};
 		let scale = self.dpi_scale();
 		if scale > 0.0
 		{
-			( pw as f32 / scale, ph as f32 / scale )
+			( pw / scale, ph / scale )
 		} else {
-			( pw as f32, ph as f32 )
+			( pw, ph )
 		}
 	}
 
@@ -235,10 +249,21 @@ impl Canvas
 	/// by `dpi_scale` at raster time, so they must NOT use this.
 	pub fn viewport_layout( &self ) -> ( f32, f32 )
 	{
+		if let Some( v ) = self.layout_viewport() { return v; }
 		let ( pw, ph ) = self.size();
 		( pw as f32, ph as f32 )
 	}
 
+	/// Inherited layout viewport, when this is a sub-canvas.
+	fn layout_viewport( &self ) -> Option<( f32, f32 )>
+	{
+		match self
+		{
+			Canvas::Software( c ) => c.layout_viewport,
+			Canvas::Gles( c )     => c.layout_viewport,
+		}
+	}
+
 	/// Resolve a stock-widget **geometry** design pixel (height, padding,
 	/// box size, gap…) through the process-wide [`crate::WidgetScaling`]
 	/// mode, into a concrete physical-pixel value for the layout tree.

src/render/setup.rs

@@ -36,6 +36,7 @@ impl SoftwareCanvas
 			font_face:       handle.face,
 			font_registry:   None,
 			dpi_scale:       1.0,
+			layout_viewport: None,
 			global_alpha:    1.0,
 			glyph_cache:     HashMap::new(),
 			clip_mask:       None,
@@ -54,6 +55,8 @@ impl SoftwareCanvas
 			font_face:      self.font_face,
 			font_registry:  self.font_registry.as_ref().map( Arc::clone ),
 			dpi_scale:      self.dpi_scale,
+			layout_viewport: Some( self.layout_viewport.unwrap_or(
+				( self.pixmap.width() as f32, self.pixmap.height() as f32 ) ) ),
 			global_alpha:   self.global_alpha,
 			glyph_cache:    HashMap::new(),
 			clip_mask:      None,