ltk: subsurface slides over overlays, axis-locked swipes, physical-space layout, touch reset on resume

Subsurfaces can now be parented to an overlay surface, not just the main surface. `SubsurfaceSpec` gains `parent: SubsurfaceParent { Main, Overlay(id) }` so a sliding panel can ride above app windows the way an overlay panel does, and an optional `gpu: bool` so content that uses the `surface-panel` backdrop-filter glass (a GLES-only pass) keeps it while sliding instead of dropping to the software rasteriser. `reconcile_subsurfaces` resolves each spec's parent independently — skipping an `Overlay` parent that is absent, unconfigured, or zero-sized — tracks the rastered size on the slot, and commits each touched parent once per frame. The ~14 GLES shader programs are compiled once into a shared `AppData::subsurface_gles_canvas` reused across every subsurface, so a lazily re-created sliding panel never recompiles them (hundreds of ms on a mobile GPU).
Vertical and horizontal swipes are now mutually exclusive: a gesture locks onto its dominant axis within the first 8 px of travel (new `SwipeAxis`) and ignores the perpendicular axis for the rest of the gesture, so a vertical swipe that drifts sideways no longer also drives the pager (and vice-versa). The upward swipe progress is no longer clamped at 1.0 — follow-the-finger panels can keep tracking the finger past the commit threshold — and a release below threshold delivers a final `progress = 0.0` cancellation pulse. A vertical swipe also no longer re-rasters the full-screen main surface on every motion event: only the overlays it drives are refreshed, while the horizontal pager (which does move the main surface) still redraws it. Re-rastering the main on every frame of a vertical drag was wasted work that stalled the loop and made the gesture feel laggy to start on a slow GPU.
Layout-affecting `Length` values (widths, paddings, gaps, widget sizes, including `Vw` / `Vh`) now resolve against the physical viewport via the new `Canvas::viewport_layout()` — the space the layout tree is actually computed in — so a `Vw(100)` fills the surface on a HiDPI (scale 2) output instead of covering half of it. Font sizes are unchanged: they still resolve against the logical viewport and are scaled at raster time. Switched column, row, spacer, wrap_grid, container, image, and vslider over to it; `img_widget` and `vslider` now document `Length::vw` / `vh` sizing and the showcase example demonstrates a viewport-relative image.
Touch gesture state is reset when the touch capability is added or removed — suspend / resume on devices that power the touchscreen down. A yanked capability never delivers the pending `up` / `cancel`, so the shared `reset_touch_state()` (also used by the `wl_touch.cancel` handler) drops the stranded `primary_touch_id` / slot state across the main surface and every overlay, keeping the first post-resume gesture clean. Also drops an accidental duplicate `on_scale_changed` call from the scale-change handler.

src/widget/container/mod.rs

@@ -275,7 +275,7 @@ impl<Msg: Clone> Container<Msg>
 	/// Return the preferred `(width, height)` accounting for padding.
 	pub fn preferred_size( &self, max_width: f32, canvas: &Canvas ) -> ( f32, f32 )
 	{
-		let vp = canvas.viewport_logical();
+		let vp = canvas.viewport_layout();
 		let em = Length::EM_BASE_DEFAULT;
 		let pad_l = self.pad_left.resolve(   vp, em );
 		let pad_r = self.pad_right.resolve(  vp, em );

src/widget/image/mod.rs

@@ -16,10 +16,13 @@ use crate::render::Canvas;
 ///
 /// ```rust,no_run
 /// # use std::sync::Arc;
-/// # use ltk::{ img_widget, Element };
+/// # use ltk::{ img_widget, Element, Length };
 /// # #[ derive( Clone ) ] enum Msg {}
 /// # fn _ex( rgba_bytes: Arc<Vec<u8>>, width: u32, height: u32 ) -> Element<Msg> {
+/// // Display at 40 % of the viewport width by 20 % of its height instead of
+/// // the source's intrinsic pixel size — scales across screens with no tweaks.
 /// img_widget( rgba_bytes, width, height )
+///     .size( Length::vw( 40.0 ), Length::vh( 20.0 ) )
 ///     .opacity( 0.8 )
 /// .into()
 /// # }
@@ -34,7 +37,7 @@ pub struct Image
 	pub height:  u32,
 	/// When `true` the image scales to fill the available width (cover mode).
 	pub cover:   bool,
-	/// Optional explicit display size in logical units.
+	/// Optional explicit display size (Length values, resolved at layout time).
 	pub display_size: Option<( Length, Length )>,
 	/// Opacity multiplier in `[0.0, 1.0]`. Default: `1.0`.
 	pub opacity: f32,
@@ -66,7 +69,8 @@ impl Image
 		self
 	}
 
-	/// Set an explicit display size in logical units.
+	/// Set an explicit display size. Accepts logical `f32` pixels or any
+	/// [`Length`] variant (e.g. `Length::vw(13.0)` for 13 % of viewport width).
 	pub fn size( mut self, width: impl Into<Length>, height: impl Into<Length> ) -> Self
 	{
 		self.display_size = Some( ( width.into(), height.into() ) );
@@ -81,15 +85,16 @@ impl Image
 	}
 
 	/// Return the preferred `(width, height)` given available `max_width`.
+	/// `canvas` is used to resolve viewport-relative [`Length`] values.
 	pub fn preferred_size( &self, max_width: f32, canvas: &Canvas ) -> (f32, f32)
 	{
-		if let Some( ( width, height ) ) = self.display_size
+		if let Some( ( w, h ) ) = &self.display_size
 		{
-			let viewport = canvas.viewport_logical();
-			return (
-				width.resolve( viewport, Length::EM_BASE_DEFAULT ).max( 0.0 ),
-				height.resolve( viewport, Length::EM_BASE_DEFAULT ).max( 0.0 ),
-			);
+			let vp  = canvas.viewport_layout();
+			let em  = Length::EM_BASE_DEFAULT;
+			let rw  = w.resolve( vp, em ).max( 0.0 );
+			let rh  = h.resolve( vp, em ).max( 0.0 );
+			return ( rw, rh );
 		}
 		if self.cover
 		{

src/widget/image/tests.rs

@@ -2,12 +2,18 @@
 // Copyright (C) 2026 Liberux Labs, S. L. <info@liberux.net>
 
 use super::*;
+use crate::render::Canvas;
 
 fn solid_rgba( w: u32, h: u32 ) -> Arc<Vec<u8>>
 {
 	Arc::new( vec![ 255u8; ( w * h * 4 ) as usize ] )
 }
 
+fn dummy_canvas() -> Canvas
+{
+	Canvas::new( 800, 600 )
+}
+
 // ── builders / defaults ───────────────────────────────────────────────────
 
 #[ test ]
@@ -57,8 +63,7 @@ fn preferred_size_default_scales_height_to_match_width_aspect()
 {
 	// 200×100 source image at max_width = 400 → scale = 2 → height 200.
 	let img = Image::new( solid_rgba( 200, 100 ), 200, 100 );
-	let canvas = Canvas::new( 480, 900 );
-	let ( w, h ) = img.preferred_size( 400.0, &canvas );
+	let ( w, h ) = img.preferred_size( 400.0, &dummy_canvas() );
 	assert_eq!( w, 400.0 );
 	assert_eq!( h, 200.0 );
 }
@@ -67,8 +72,7 @@ fn preferred_size_default_scales_height_to_match_width_aspect()
 fn preferred_size_with_explicit_size_wins_over_aspect_logic()
 {
 	let img = Image::new( solid_rgba( 200, 100 ), 200, 100 ).size( 50.0, 25.0 );
-	let canvas = Canvas::new( 480, 900 );
-	assert_eq!( img.preferred_size( 1000.0, &canvas ), ( 50.0, 25.0 ) );
+	assert_eq!( img.preferred_size( 1000.0, &dummy_canvas() ), ( 50.0, 25.0 ) );
 }
 
 #[ test ]
@@ -84,8 +88,7 @@ fn preferred_size_cover_mode_keeps_aspect_ratio()
 {
 	// 100×50 source in cover mode at max_width = 300 → height = 300 * (50/100) = 150.
 	let img = Image::new( solid_rgba( 100, 50 ), 100, 50 ).cover();
-	let canvas = Canvas::new( 480, 900 );
-	let ( w, h ) = img.preferred_size( 300.0, &canvas );
+	let ( w, h ) = img.preferred_size( 300.0, &dummy_canvas() );
 	assert_eq!( w, 300.0 );
 	assert_eq!( h, 150.0 );
 }
@@ -98,8 +101,8 @@ fn preferred_size_cover_and_default_match_for_uniform_scaling()
 	// when the source is taller than wide.
 	let normal  = Image::new( solid_rgba( 100, 50 ), 100, 50 );
 	let covered = Image::new( solid_rgba( 100, 50 ), 100, 50 ).cover();
-	let canvas = Canvas::new( 480, 900 );
-	assert_eq!( normal.preferred_size( 200.0, &canvas ), covered.preferred_size( 200.0, &canvas ) );
+	let c = dummy_canvas();
+	assert_eq!( normal.preferred_size( 200.0, &c ), covered.preferred_size( 200.0, &c ) );
 }
 
 // ── Arc sharing ───────────────────────────────────────────────────────────

src/widget/vslider/mod.rs

@@ -65,9 +65,9 @@ pub struct VSlider<Msg: Clone>
 	/// Current value in `[0.0, 1.0]`. `0.0` paints no fill; `1.0` fills the
 	/// whole pill.
 	pub value:     f32,
-	/// Fixed width of the pill in logical units. Defaults to 56 px.
+	/// Width of the pill. Defaults to 56 px; accepts any [`Length`].
 	pub width:     Length,
-	/// Fixed height of the pill in logical units. Defaults to 160 px.
+	/// Height of the pill. Defaults to 160 px; accepts any [`Length`].
 	pub height:    Length,
 	/// Callback invoked with the new value when the slider is tapped or
 	/// dragged. `Arc` (not `Box`) so the layout pass can clone it into the
@@ -117,9 +117,10 @@ impl<Msg: Clone> VSlider<Msg>
 		self
 	}
 
-	/// Override the fixed pill `(width, height)` in logical units. Both are
-	/// clamped to a minimum of `2.0` after viewport-relative values resolve,
-	/// so a rounded pill can always be drawn.
+	/// Override the pill size. Accepts logical `f32` pixels or any [`Length`]
+	/// variant (e.g. `Length::vw(16.0)` for 16 % of the viewport width). Both
+	/// are clamped to a minimum of `2.0` after viewport-relative values
+	/// resolve, so a rounded pill can always be drawn.
 	pub fn size( mut self, width: impl Into<Length>, height: impl Into<Length> ) -> Self
 	{
 		self.width  = width.into();
@@ -138,11 +139,11 @@ impl<Msg: Clone> VSlider<Msg>
 	/// the type-level docs on intrinsic sizing.
 	pub fn preferred_size( &self, _max_width: f32, canvas: &Canvas ) -> (f32, f32)
 	{
-		let viewport = canvas.viewport_logical();
-		(
-			self.width.resolve( viewport, Length::EM_BASE_DEFAULT ).max( 2.0 ),
-			self.height.resolve( viewport, Length::EM_BASE_DEFAULT ).max( 2.0 ),
-		)
+		let vp = canvas.viewport_layout();
+		let em = Length::EM_BASE_DEFAULT;
+		let w  = self.width.resolve( vp, em ).max( 2.0 );
+		let h  = self.height.resolve( vp, em ).max( 2.0 );
+		( w, h )
 	}
 
 	/// Compute the value `[0.0, 1.0]` from a tap/drag y position within `rect`.