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.