row, button, widget: distribute a row's width shortfall instead of overflowing, elide button labels into the rect they are granted, icon_size takes a Length

Row deficit distribution (layout/row.rs). `Row` knew how to hand out leftover width but had no notion of a shortfall: `leftover` was floored at zero and every child was laid out at its preferred width, so a cluster wider than its rect simply overflowed — and symmetrically, because the no-spacer branch centres the block, which is why both end labels of a segmented control were clipped at once rather than only the trailing one. The shortfall now comes off the widest children first, by water filling: `width_cap` sorts the flexible widths and returns the largest per-child cap `c` for which `sum( min( w, c ) ) <= available`, so a long title absorbs the whole deficit while an icon button beside it keeps its size, and equal siblings — a segmented control — share it evenly. A uniform scale-down, which was the first attempt, was wrong precisely there: it thinned a header's back arrow along with the title it sat next to. Spacers and flex children stay out of the flexible set, so a pinned gap keeps the width it was given — an explicit spacer is a decision, not slack — and `Row::no_shrink()` opts a row out entirely, for strips deliberately wider than their viewport such as a carousel rail meant to be scrolled. `align_right` and the centring branch now measure the laid width rather than the preferred one, so a shrunken row is positioned against what it actually occupies.
Button label elision (widget/button/mod.rs, widget/mod.rs, widget/list_item/mod.rs). Shrinking a row is only half the fix: a leaf handed less width than it asked for still painted its full string, so the deficit came out clipped instead of truncated. `draw_text_button` now elides the label against the rect the layout granted, less the horizontal padding, for all three variants. The truncation rule moves out of `ListItem`, where it was a private helper, into a single crate-internal `widget::elide` the two share. `Text` keeps its own inline copy for now: it measures through an optional font override that this signature does not carry, and folding the two together is a separate change.
icon_size as a Length (widget/button/mod.rs). `Button::icon_size` was the one geometry setter that ignored the widget-scaling mode — it took a bare `f32`, pinned it, and used `0.0` as the "unset" sentinel. It now takes `impl Into<Length>` behind an `Option` and resolves through `Canvas::resolve_geom`, the way `height`, `width` and `font_size` already do. A bare number still means `Length::px`, so every existing call keeps its exact size; what it adds is `Length::widget( n )`, which follows the active mode the way stock icons do. Without it a back arrow pinned at 21 px sat next to a `list_item` chevron that fluid sizing had grown well past 21, and read as visibly smaller on the same row.

src/widget/button/mod.rs

@@ -69,7 +69,7 @@ pub struct Button<Msg: Clone>
 	/// Width and height in pixels for icon buttons. The `0.0` default
 	/// follows the process [`crate::WidgetScaling`] mode (via
 	/// [`crate::Canvas::geom_px`]); any positive value pins an explicit size.
-	pub( crate ) icon_size: f32,
+	pub( crate ) icon_size: Option<Length>,
 	/// Optional label font size for text buttons. `None` uses the theme's
 	/// default (`theme::FONT_SIZE`); a [`Length`] scales the label with the
 	/// surface (e.g. `Length::vmin( 2.2 ).clamp( 14.0, 22.0 )`).
@@ -115,7 +115,7 @@ impl<Msg: Clone> Button<Msg>
 			on_long_press: None,
 			on_drag_start: None,
 			variant:       ButtonVariant::Primary,
-			icon_size:     0.0,
+			icon_size:     None,
 			font_size:     None,
 			height:        None,
 			width:         None,
@@ -153,7 +153,7 @@ impl<Msg: Clone> Button<Msg>
 			on_long_press: None,
 			on_drag_start: None,
 			variant:       ButtonVariant::Tertiary,
-			icon_size:     0.0,
+			icon_size:     None,
 			font_size:     None,
 			height:        None,
 			width:         None,
@@ -237,10 +237,16 @@ impl<Msg: Clone> Button<Msg>
 		self
 	}
 
-	/// Set the display size (width = height) for icon buttons in pixels.
-	pub fn icon_size( mut self, size: f32 ) -> Self
+	/// Set the display size (width = height) for icon buttons.
+	///
+	/// A bare `f32` is `Length::px`, which pins the icon at that many canvas
+	/// pixels. Pass `Length::widget( n )` to have it follow the active
+	/// widget-scaling mode, which is what stock icons — a `list_item`
+	/// disclosure arrow, for one — already do, so the two match on a surface
+	/// where fluid sizing has grown them.
+	pub fn icon_size( mut self, size: impl Into<Length> ) -> Self
 	{
-		self.icon_size = size;
+		self.icon_size = Some( size.into() );
 		self
 	}
 
@@ -293,11 +299,17 @@ impl<Msg: Clone> Button<Msg>
 			.unwrap_or_else( || canvas.geom_px( theme::HEIGHT ) )
 	}
 
-	/// Resolve the icon-button size: a positive [`Self::icon_size`] pins it,
-	/// the `0.0` sentinel follows the widget-scaling mode.
+	/// Resolve the icon-button size. Unset follows the widget-scaling mode
+	/// through `theme::HEIGHT`; an explicit value resolves like any other
+	/// geometry `Length`, so `Length::px` pins a physical size while
+	/// `Length::widget` tracks the surface the way stock icons do.
 	fn resolved_icon_size( &self, canvas: &Canvas ) -> f32
 	{
-		if self.icon_size > 0.0 { self.icon_size } else { canvas.geom_px( theme::HEIGHT ) }
+		match self.icon_size
+		{
+			Some( l ) => canvas.resolve_geom( l ),
+			None      => canvas.geom_px( theme::HEIGHT ),
+		}
 	}
 
 	/// Assign a stable identifier for focus management.
@@ -389,6 +401,9 @@ impl<Msg: Clone> Button<Msg>
 		let is_disabled = self.on_press.is_none();
 		let fs          = self.label_font_size( canvas );
 		let text_y      = rect.y + (rect.height + fs) / 2.0 - 2.0;
+		// `rect` is what the layout granted, which is not always the
+		// preferred width: a row that cannot fit its children shrinks them.
+		let label       = super::elide( canvas, label, fs, rect.width - canvas.geom_px( theme::PAD_H ) * 2.0 );
 
 		match self.variant
 		{
@@ -412,9 +427,9 @@ impl<Msg: Clone> Button<Msg>
 						theme::RADIUS + theme::FOCUS_W + 2.0,
 					);
 				}
-				let text_w = canvas.measure_text( label, fs );
+				let text_w = canvas.measure_text( &label, fs );
 				canvas.draw_text(
-					label,
+					&label,
 					rect.x + (rect.width - text_w) / 2.0,
 					text_y,
 					fs,
@@ -438,9 +453,9 @@ impl<Msg: Clone> Button<Msg>
 						theme::RADIUS + theme::FOCUS_W + 2.0,
 					);
 				}
-				let text_w = canvas.measure_text( label, fs );
+				let text_w = canvas.measure_text( &label, fs );
 				canvas.draw_text(
-					label,
+					&label,
 					rect.x + (rect.width - text_w) / 2.0,
 					text_y,
 					fs,
@@ -455,9 +470,9 @@ impl<Msg: Clone> Button<Msg>
 					let ring = rect.expand( 2.0 );
 					canvas.stroke_rect( ring, theme::focus_color(), theme::FOCUS_W, theme::RADIUS );
 				}
-				let text_w = canvas.measure_text( label, fs );
+				let text_w = canvas.measure_text( &label, fs );
 				canvas.draw_text(
-					label,
+					&label,
 					rect.x + (rect.width - text_w) / 2.0,
 					text_y,
 					fs,