Resolve fluid geometry bounds in logical space and let the runtime set the density from the buffer scale

Stock widgets size themselves with `Length::fluid`, whose auto-clamp `[0.7·px, 1.5·px]` is documented as logical px, yet `Canvas::resolve_geom` resolved it against the physical viewport and applied the bounds to the physical value. On any HiDPI surface whose physical short side exceeds 618 px (412 × 1.5) the cap engaged: a 1080 × 1920 phone at scale 2 got its geometry at 1.5 physical px per design px, that is 0.75 logical, while fonts, resolved by `font_px` against the logical viewport and scaled at raster time, kept growing to 2.62. Rows, paddings and icons came out at 60 % of their intended size with the text overflowing them; at scale 1 physical and logical coincide, so a winit session never showed it.
`Length::resolve_physical` takes the buffer scale and multiplies `min_px` / `max_px` by it before clamping; `resolve_with_density` delegates to it with scale 1. `Canvas::resolve_geom` passes its `dpi_scale`, so every stock-widget geometry shares the surface factor of the fonts, and `overlays_reconcile` does the same for `OverlaySpec::size`, which had the same mix. A zero scale falls back to unscaled bounds.
The event loop now calls `set_density( sf )` right before `App::on_scale_changed`, both on `configure` and on `scale_factor_changed` of the main surface, so `Length::dp( n )` means n logical px in every app without plumbing; an app wanting a DPI-derived density still overrides it from that callback, and crustace already sets the same value there. Two tests cover the clamp at scale 2 and the geometry/font parity on a 1080 × 2460 canvas.

src/types.rs

@@ -639,10 +639,22 @@ impl Length
 	/// what [`crate::Canvas`]-routed resolution calls with the canvas'
 	/// own density.
 	pub fn resolve_with_density( &self, viewport: ( f32, f32 ), em_base: f32, density: f32 ) -> f32
+	{
+		self.resolve_physical( viewport, em_base, density, 1.0 )
+	}
+
+	/// [`Self::resolve_with_density`] against a **physical** viewport
+	/// (`dpi_scale` physical px per logical px). The bounds are logical
+	/// px, so they are scaled to physical before they apply; otherwise a
+	/// `fluid` value hits its cap on any HiDPI surface whose physical
+	/// short side exceeds the cap, while fonts (resolved in logical space)
+	/// keep growing.
+	pub fn resolve_physical( &self, viewport: ( f32, f32 ), em_base: f32, density: f32, dpi_scale: f32 ) -> f32
 	{
 		let raw = self.base.resolve( viewport, em_base, density );
-		let lo  = self.min_px;
-		let hi  = self.max_px;
+		let k   = if dpi_scale > 0.0 { dpi_scale } else { 1.0 };
+		let lo  = self.min_px.map( |v| v * k );
+		let hi  = self.max_px.map( |v| v * k );
 		// If both bounds present, normalise their order so swapped args
 		// don't produce NaN out of f32::clamp.
 		let ( lo, hi ) = match ( lo, hi )
@@ -732,16 +744,17 @@ pub fn text_scale() -> f32
 static DENSITY_BITS: AtomicU32 = AtomicU32::new( 1.0_f32.to_bits() );
 
 /// Set the process-wide pixel density used by [`Length::dp`] (the
-/// [`WidgetScaling::Physical`] mode). Typically derived from the output's
-/// physical DPI so `dp` sizes stay physically constant across displays.
-/// Default: `1.0`.
+/// [`WidgetScaling::Physical`] mode). The runtime sets it to the main
+/// surface's buffer scale right before [`crate::App::on_scale_changed`],
+/// so `dp( n )` is `n` logical px by default; an app that wants a
+/// DPI-derived density overrides it from that callback. Default: `1.0`.
 pub fn set_density( d: f32 )
 {
 	DENSITY_BITS.store( d.max( 0.0 ).to_bits(), Ordering::Relaxed );
 }
 
 /// Current pixel density — the factor [`Length::dp`] multiplies its design
-/// pixels by. `1.0` until [`set_density`] is called.
+/// pixels by. `1.0` until the runtime or [`set_density`] sets it.
 pub fn density() -> f32
 {
 	f32::from_bits( DENSITY_BITS.load( Ordering::Relaxed ) )
@@ -946,6 +959,20 @@ mod length_tests
 		assert_eq!( p, l );
 	}
 
+	#[ test ]
+	fn fluid_bounds_scale_with_dpi_on_a_physical_viewport()
+	{
+		// 1080 × 2460 at scale 2: the physical short side is 2.62 × the
+		// reference, the cap is 1.5 × 48 logical = 144 physical, so the
+		// unclamped 125.8 stands.
+		let v = Length::fluid( 48.0 ).resolve_physical( ( 1080.0, 2460.0 ), 16.0, 1.0, 2.0 );
+		assert!( ( v - 48.0 / 412.0 * 1080.0 ).abs() < 1e-3 );
+		// Same surface treated as scale 1 hits the cap in physical px.
+		assert_eq!( Length::fluid( 48.0 ).resolve_physical( ( 1080.0, 2460.0 ), 16.0, 1.0, 1.0 ), 72.0 );
+		// A zero scale falls back to unscaled bounds instead of collapsing to 0.
+		assert_eq!( Length::fluid( 48.0 ).resolve_physical( ( 1080.0, 2460.0 ), 16.0, 1.0, 0.0 ), 72.0 );
+	}
+
 	#[ test ]
 	fn dp_is_identity_at_default_density()
 	{