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/layout/row.rs

@@ -49,6 +49,9 @@ pub struct Row<Msg: Clone>
 	pub( crate ) align_right: bool,
 	pub( crate ) align_top:   bool,
 	pub( crate ) fill_height: bool,
+	/// When false the row keeps every child at its preferred width even if
+	/// the cluster overflows. Default true.
+	pub( crate ) shrink:      bool,
 }
 
 impl<Msg: Clone> Row<Msg>
@@ -63,6 +66,7 @@ impl<Msg: Clone> Row<Msg>
 			align_right: false,
 			align_top:   false,
 			fill_height: false,
+			shrink:      true,
 		}
 	}
 
@@ -101,6 +105,19 @@ impl<Msg: Clone> Row<Msg>
 		canvas.resolve_geom( self.padding )
 	}
 
+	/// Keep every child at its preferred width even when the cluster does not
+	/// fit, letting it overflow the row's rect.
+	///
+	/// The default distributes the shortfall instead, which is what a row of
+	/// labelled buttons wants. Opt out for strips that are deliberately wider
+	/// than their viewport — a carousel rail, a tray meant to be scrolled —
+	/// where shrinking to fit would defeat the point.
+	pub fn no_shrink( mut self ) -> Self
+	{
+		self.shrink = false;
+		self
+	}
+
 	/// Push the content block to the right edge of the available width.
 	pub fn align_right( mut self ) -> Self
 	{
@@ -231,8 +248,28 @@ impl<Msg: Clone> Row<Msg>
 			} )
 			.sum();
 
-		let inner_w     = ( rect.width - pad * 2.0 ).max( 0.0 );
-		let leftover    = ( inner_w - fixed_w - gaps ).max( 0.0 );
+		let inner_w = ( rect.width - pad * 2.0 ).max( 0.0 );
+
+		// Deficit distribution: cap the widest children first, so a long
+		// label gives up its width before a neighbouring icon does. A uniform
+		// scale-down would thin a header's back arrow along with its title.
+		// Spacers keep the width they were pinned to — an explicit gap is a
+		// decision, not slack.
+		let mut flexible: Vec<f32> = self.children.iter().zip( sizes.iter() )
+			.filter( |( c, _ )| !matches!( c, Element::Flex( _ ) | Element::Spacer( _ ) ) )
+			.map( |( _, ( w, _ ) )| *w )
+			.collect();
+		let flexible_w: f32 = flexible.iter().sum();
+		let rigid_w = ( fixed_w - flexible_w ).max( 0.0 );
+		let cap = if self.shrink
+		{
+			width_cap( &mut flexible, ( inner_w - gaps - rigid_w ).max( 0.0 ) )
+		} else {
+			f32::INFINITY
+		};
+
+		let laid_w      = rigid_w + flexible.iter().map( |w| w.min( cap ) ).sum::<f32>();
+		let leftover    = ( inner_w - laid_w - gaps ).max( 0.0 );
 		let has_spacers = total_weight > 0;
 
 		let ( start_x, flex_unit ) = if has_spacers
@@ -243,11 +280,11 @@ impl<Msg: Clone> Row<Msg>
 		}
 		else if self.align_right
 		{
-			( rect.x + rect.width - ( fixed_w + gaps ) - pad, 0.0 )
+			( rect.x + rect.width - ( laid_w + gaps ) - pad, 0.0 )
 		}
 		else
 		{
-			( rect.x + ( rect.width - fixed_w - gaps ) / 2.0, 0.0 )
+			( rect.x + ( rect.width - laid_w - gaps ) / 2.0, 0.0 )
 		};
 
 		let mut x = start_x;
@@ -262,7 +299,7 @@ impl<Msg: Clone> Row<Msg>
 					None       => flex_unit * s.weight as f32,
 				},
 				Element::Flex( f )   => flex_unit * f.weight as f32,
-				_                    => w,
+				_                    => w.min( cap ),
 			};
 			let ( y, height ) = if self.fill_height && !matches!( child, Element::Spacer( _ ) )
 			{
@@ -291,10 +328,40 @@ impl<Msg: Clone> Row<Msg>
 			align_right: self.align_right,
 			align_top:   self.align_top,
 			fill_height: self.fill_height,
+			shrink:      self.shrink,
 		}
 	}
 }
 
+/// Largest per-child width `c` for which `sum( min( w, c ) ) <= available`.
+///
+/// Water filling: the widest children give up width first and the narrow ones
+/// are left alone until the wide ones have been levelled down to them. In a
+/// header that means the title absorbs the whole shortfall while the back
+/// arrow beside it keeps its size. Sorts `widths` in place; returns infinity
+/// when everything already fits.
+fn width_cap( widths: &mut [f32], available: f32 ) -> f32
+{
+	if widths.iter().sum::<f32>() <= available
+	{
+		return f32::INFINITY;
+	}
+	widths.sort_by( |a, b| a.partial_cmp( b ).unwrap_or( std::cmp::Ordering::Equal ) );
+
+	let mut prefix = 0.0_f32;
+	for ( i, w ) in widths.iter().enumerate()
+	{
+		let remaining = ( widths.len() - i ) as f32;
+		let cap = ( available - prefix ) / remaining;
+		if cap <= *w
+		{
+			return cap.max( 0.0 );
+		}
+		prefix += *w;
+	}
+	0.0
+}
+
 /// Create an empty row layout.
 pub fn row<Msg: Clone>() -> Row<Msg>
 {
@@ -318,6 +385,75 @@ mod tests
 
 	fn make_canvas() -> Canvas { Canvas::new( 800, 600 ) }
 
+	/// Children whose preferred widths overflow the rect are scaled down to
+	/// fit, rather than spilling symmetrically out of it.
+	#[ test ]
+	fn overflowing_children_shrink_to_fit()
+	{
+		let canvas = make_canvas();
+		let build  = || row::<()>()
+			.spacing( 0.0 )
+			.padding( 0.0 )
+			.push( crate::toggle( false ) )
+			.push( crate::toggle( false ) );
+
+		let natural = build().preferred_size( 10_000.0, &canvas ).0;
+		assert!( natural > 0.0, "toggles must report a width for this to test anything" );
+		let rect = Rect { x: 0.0, y: 0.0, width: natural / 2.0, height: 48.0 };
+
+		let laid: f32 = build().layout( rect, &canvas )
+			.iter().map( |( r, _ )| r.width ).sum();
+		assert!( laid <= rect.width + 0.5, "laid out {laid} into {}", rect.width );
+
+		let kept: f32 = build().no_shrink().layout( rect, &canvas )
+			.iter().map( |( r, _ )| r.width ).sum();
+		assert!( kept > rect.width, "no_shrink kept {kept}, expected wider than {}", rect.width );
+	}
+
+	/// The shortfall comes off the widest child first: a narrow neighbour —
+	/// a header's back arrow next to its title — keeps its size.
+	#[ test ]
+	fn narrow_child_survives_a_wide_neighbour()
+	{
+		let mut w = vec![ 20.0_f32, 300.0 ];
+		let cap   = width_cap( &mut w, 200.0 );
+		assert_eq!( 20.0_f32.min( cap ), 20.0 );
+		assert_eq!( 300.0_f32.min( cap ), 180.0 );
+	}
+
+	/// Equal children share the shortfall equally, which is what a segmented
+	/// control wants.
+	#[ test ]
+	fn equal_children_shrink_equally()
+	{
+		let mut w = vec![ 100.0_f32, 100.0, 100.0 ];
+		let cap   = width_cap( &mut w, 150.0 );
+		assert_eq!( cap, 50.0 );
+	}
+
+	#[ test ]
+	fn nothing_shrinks_when_it_all_fits()
+	{
+		let mut w = vec![ 10.0_f32, 20.0 ];
+		assert_eq!( width_cap( &mut w, 100.0 ), f32::INFINITY );
+	}
+
+	/// Pinned spacers are an explicit gap, not slack, so the shrink pass
+	/// leaves them alone.
+	#[ test ]
+	fn pinned_spacers_keep_their_width_when_shrinking()
+	{
+		let canvas = make_canvas();
+		let r = row::<()>()
+			.spacing( 0.0 )
+			.padding( 0.0 )
+			.push( crate::spacer().width( 60.0 ).height( 10.0 ) )
+			.push( crate::toggle( false ) );
+		let rect  = Rect { x: 0.0, y: 0.0, width: 70.0, height: 48.0 };
+		let cells = r.layout( rect, &canvas );
+		assert_eq!( cells[ 0 ].0.width, 60.0 );
+	}
+
 	#[ test ]
 	fn align_right_returns_full_max_width()
 	{