iris: separate layout allocation from lengths

benches/message_list.rs

@@ -140,7 +140,7 @@ fn bench_input_grows(n: usize, lines: usize) {
     let input_area = rsc.ui.widgets.add_strong(Sized {
         inner: input_rect.any(),
         x: None,
-        y: Some(abs(line_height)),
+        y: Some(abs(line_height).into()),
     });
 
     let input_area_weak = input_area.weak();
@@ -162,7 +162,7 @@ fn bench_input_grows(n: usize, lines: usize) {
     let mut total_moves = 0u64;
     for line in 1..=lines {
         rsc.ui.widgets.get_mut(&input_area_weak).unwrap().y =
-            Some(abs(line_height * (line + 1) as f32));
+            Some(abs(line_height * (line + 1) as f32).into());
         let start = Instant::now();
         render.update(&root, &mut rsc);
         total += start.elapsed();
@@ -253,7 +253,7 @@ fn bench_expand_holds_edge(n: usize, growths: usize) {
             let sized = rsc.ui.widgets.add_strong(Sized {
                 inner: rect.any(),
                 x: None,
-                y: Some(abs(40.0)),
+                y: Some(abs(40.0).into()),
             });
             growable = Some(sized.weak());
             list.push_back(LazyItem::new(i as u64, sized.any()));
@@ -287,7 +287,7 @@ fn bench_expand_holds_edge(n: usize, growths: usize) {
                 .unwrap()
                 .note_tap(top + 1.0);
         }
-        rsc.ui.widgets.get_mut(&growable).unwrap().y = Some(abs(height));
+        rsc.ui.widgets.get_mut(&growable).unwrap().y = Some(abs(height).into());
         let start = Instant::now();
         render.update(&root, &mut rsc);
         total += start.elapsed();

core/src/orientation/len.rs

@@ -3,10 +3,13 @@ use crate::{UiNum, util::impl_op};
 
 #[derive(Debug, Default, Clone, Copy, PartialEq)]
 pub struct Size {
-    pub x: Len,
-    pub y: Len,
+    pub x: LayoutLen,
+    pub y: LayoutLen,
 }
 
+/// A length resolved from physical pixels, density-independent pixels, and a
+/// fraction of a reference length. Unlike [`LayoutLen`], it carries no claim
+/// on space left over by a layout container.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Len {
     /// Physical pixels -- a raw device pixel, unaffected by the display's
@@ -17,6 +20,15 @@ pub struct Len {
     pub abs: f32,
     pub dp: f32,
     pub rel: f32,
+}
+
+/// A widget length plus its proportional claim on the space left after fixed
+/// and relative lengths have been allocated.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct LayoutLen {
+    pub abs: f32,
+    pub dp: f32,
+    pub rel: f32,
     pub rest: f32,
 }
 
@@ -26,8 +38,25 @@ impl<N: UiNum> From<N> for Len {
     }
 }
 
-impl<Nx: UiNum, Ny: UiNum> From<(Nx, Ny)> for Size {
-    fn from((x, y): (Nx, Ny)) -> Self {
+impl<N: UiNum> From<N> for LayoutLen {
+    fn from(value: N) -> Self {
+        Self::abs(value.to_f32())
+    }
+}
+
+impl From<Len> for LayoutLen {
+    fn from(value: Len) -> Self {
+        Self {
+            abs: value.abs,
+            dp: value.dp,
+            rel: value.rel,
+            rest: 0.0,
+        }
+    }
+}
+
+impl<X: Into<LayoutLen>, Y: Into<LayoutLen>> From<(X, Y)> for Size {
+    fn from((x, y): (X, Y)) -> Self {
         Self {
             x: x.into(),
             y: y.into(),
@@ -35,41 +64,47 @@ impl<Nx: UiNum, Ny: UiNum> From<(Nx, Ny)> for Size {
     }
 }
 
+impl From<LayoutLen> for Size {
+    fn from(value: LayoutLen) -> Self {
+        Self { x: value, y: value }
+    }
+}
+
 impl From<Len> for Size {
     fn from(value: Len) -> Self {
-        Self { x: value, y: value }
+        Self::from(LayoutLen::from(value))
     }
 }
 
 impl Size {
     pub const ZERO: Self = Self {
-        x: Len::ZERO,
-        y: Len::ZERO,
+        x: LayoutLen::ZERO,
+        y: LayoutLen::ZERO,
     };
 
     pub const REST: Self = Self {
-        x: Len::REST,
-        y: Len::REST,
+        x: LayoutLen::REST,
+        y: LayoutLen::REST,
     };
 
     pub fn abs(v: Vec2) -> Self {
         Self {
-            x: Len::abs(v.x),
-            y: Len::abs(v.y),
+            x: LayoutLen::abs(v.x),
+            y: LayoutLen::abs(v.y),
         }
     }
 
     pub fn rel(v: Vec2) -> Self {
         Self {
-            x: Len::rel(v.x),
-            y: Len::rel(v.y),
+            x: LayoutLen::rel(v.x),
+            y: LayoutLen::rel(v.y),
         }
     }
 
     pub fn rest(v: Vec2) -> Self {
         Self {
-            x: Len::rest(v.x),
-            y: Len::rest(v.y),
+            x: LayoutLen::rest(v.x),
+            y: LayoutLen::rest(v.y),
         }
     }
 
@@ -80,7 +115,7 @@ impl Size {
         }
     }
 
-    pub fn from_axis(axis: Axis, aligned: Len, ortho: Len) -> Self {
+    pub fn from_axis(axis: Axis, aligned: LayoutLen, ortho: LayoutLen) -> Self {
         match axis {
             Axis::X => Self {
                 x: aligned,
@@ -93,7 +128,7 @@ impl Size {
         }
     }
 
-    pub fn axis(&self, axis: Axis) -> Len {
+    pub fn axis(&self, axis: Axis) -> LayoutLen {
         match axis {
             Axis::X => self.x,
             Axis::Y => self.y,
@@ -101,7 +136,7 @@ impl Size {
     }
 }
 
-impl Len {
+impl LayoutLen {
     pub const ZERO: Self = Self {
         abs: 0.0,
         dp: 0.0,
@@ -119,7 +154,7 @@ impl Len {
     /// Resolves to a `UiScalar`, folding `dp` into `abs` pixels against
     /// `density` (physical pixels per dp -- 1.0 on a desktop or an
     /// unscaled display, `content_scale` on Android; see `dp`'s field
-    /// doc). Every other component of `Len` is already resolution-
+    /// doc). Every other component of `LayoutLen` is already resolution-
     /// independent (`rel` is a fraction of the parent; `rest` becomes a
     /// fraction too, below), so `density` only ever touches this one term.
     pub fn apply_rest(&self, density: f32) -> UiScalar {
@@ -129,11 +164,11 @@ impl Len {
         }
     }
 
-    /// The same fold as [`Self::apply_rest`] but staying a `Len`, so
+    /// The same fold as [`Self::apply_rest`] but staying a `LayoutLen`, so
     /// `rest` survives: `dp` becomes physical pixels and every other
     /// component is left alone.
     ///
-    /// **A `Len` a widget *reports* must have been through this.** `dp` is
+    /// **A `LayoutLen` a widget *reports* must have been through this.** `dp` is
     /// an input unit -- a number the widget author wrote -- and the
     /// containers that consume a reported length read `abs`/`rel`/`rest`
     /// directly (`Span::draw`'s placement arithmetic, `Pad`'s addition),
@@ -186,35 +221,68 @@ impl Len {
     }
 }
 
+impl Len {
+    pub const ZERO: Self = Self {
+        abs: 0.0,
+        dp: 0.0,
+        rel: 0.0,
+    };
+
+    pub fn abs(abs: impl UiNum) -> Self {
+        Self {
+            abs: abs.to_f32(),
+            dp: 0.0,
+            rel: 0.0,
+        }
+    }
+
+    pub fn dp(dp: impl UiNum) -> Self {
+        Self {
+            abs: 0.0,
+            dp: dp.to_f32(),
+            rel: 0.0,
+        }
+    }
+
+    pub fn rel(rel: impl UiNum) -> Self {
+        Self {
+            abs: 0.0,
+            dp: 0.0,
+            rel: rel.to_f32(),
+        }
+    }
+
+    pub const fn fold_dp(self, density: f32) -> Self {
+        Self {
+            abs: self.abs + self.dp * density,
+            dp: 0.0,
+            rel: self.rel,
+        }
+    }
+
+    pub const fn resolve(self, density: f32) -> UiScalar {
+        let folded = self.fold_dp(density);
+        UiScalar {
+            rel: folded.rel,
+            abs: folded.abs,
+        }
+    }
+}
+
 pub mod len_fns {
     use super::*;
 
     pub fn abs(abs: impl UiNum) -> Len {
-        Len {
-            abs: abs.to_f32(),
-            dp: 0.0,
-            rel: 0.0,
-            rest: 0.0,
-        }
+        Len::abs(abs)
     }
     pub fn dp(dp: impl UiNum) -> Len {
-        Len {
-            abs: 0.0,
-            dp: dp.to_f32(),
-            rel: 0.0,
-            rest: 0.0,
-        }
+        Len::dp(dp)
     }
     pub fn rel(rel: impl UiNum) -> Len {
-        Len {
-            abs: 0.0,
-            dp: 0.0,
-            rel: rel.to_f32(),
-            rest: 0.0,
-        }
+        Len::rel(rel)
     }
-    pub fn rest(ratio: impl UiNum) -> Len {
-        Len {
+    pub fn rest(ratio: impl UiNum) -> LayoutLen {
+        LayoutLen {
             abs: 0.0,
             dp: 0.0,
             rel: 0.0,
@@ -223,25 +291,49 @@ pub mod len_fns {
     }
 }
 
-impl_op!(Len Add add; abs dp rel rest);
-impl_op!(Len Sub sub; abs dp rel rest);
+impl_op!(LayoutLen Add add; abs dp rel rest);
+impl_op!(LayoutLen Sub sub; abs dp rel rest);
+impl_op!(Len Add add; abs dp rel);
+impl_op!(Len Sub sub; abs dp rel);
+
+impl std::ops::Add<Len> for LayoutLen {
+    type Output = Self;
+
+    fn add(self, rhs: Len) -> Self::Output {
+        self + Self::from(rhs)
+    }
+}
+
+impl std::ops::Sub<Len> for LayoutLen {
+    type Output = Self;
+
+    fn sub(self, rhs: Len) -> Self::Output {
+        self - Self::from(rhs)
+    }
+}
 
 impl_op!(Size Add add; x y);
 impl_op!(Size Sub sub; x y);
 
-impl Default for Len {
+impl Default for LayoutLen {
     fn default() -> Self {
         Self::rest(1.0)
     }
 }
 
+impl Default for Len {
+    fn default() -> Self {
+        Self::ZERO
+    }
+}
+
 impl std::fmt::Display for Size {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "({}, {})", self.x, self.y)
     }
 }
 
-impl std::fmt::Display for Len {
+impl std::fmt::Display for LayoutLen {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         if self.abs != 0.0 {
             write!(f, "{} abs;", self.abs)?;
@@ -258,3 +350,55 @@ impl std::fmt::Display for Len {
         Ok(())
     }
 }
+
+impl std::fmt::Display for Len {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        if self.abs != 0.0 {
+            write!(f, "{} abs;", self.abs)?;
+        }
+        if self.dp != 0.0 {
+            write!(f, "{} dp;", self.dp)?;
+        }
+        if self.rel != 0.0 {
+            write!(f, "{} rel;", self.rel)?;
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn an_ordinary_length_enters_layout_without_claiming_rest() {
+        let layout = LayoutLen::from(Len {
+            abs: 3.0,
+            dp: 4.0,
+            rel: 0.5,
+        });
+        assert_eq!(layout.abs, 3.0);
+        assert_eq!(layout.dp, 4.0);
+        assert_eq!(layout.rel, 0.5);
+        assert_eq!(layout.rest, 0.0);
+    }
+
+    #[test]
+    fn ordinary_and_layout_lengths_keep_their_own_defaults() {
+        assert_eq!(Len::default(), Len::ZERO);
+        assert_eq!(LayoutLen::default(), LayoutLen::REST);
+    }
+
+    #[test]
+    fn adding_an_ordinary_length_preserves_a_layout_claim() {
+        assert_eq!(
+            LayoutLen::rest(2) + Len::dp(8),
+            LayoutLen {
+                abs: 0.0,
+                dp: 8.0,
+                rel: 0.0,
+                rest: 2.0,
+            }
+        );
+    }
+}

core/src/ui/painter.rs

@@ -1,7 +1,7 @@
 use crate::{
-    Axis, Len, MoveOffset, PaintId, RegionAlign, RenderedText, Size, StrongWidget, TextHandle,
-    TextResources, TextureHandle, UiData, UiRegion, UiRenderState, UiRsc, UiScalar, UiVec2,
-    WidgetId,
+    Axis, LayoutLen, MoveOffset, PaintId, RegionAlign, RenderedText, Size, StrongWidget,
+    TextHandle, TextResources, TextureHandle, UiData, UiRegion, UiRenderState, UiRsc, UiScalar,
+    UiVec2, WidgetId,
     render::{
         Drawn, GlyphPrimitive, IMAGE_BINDING, Mask, MaskIdx, MoveIdx, NOT_DRAWN, Primitive,
         PrimitiveHandle, PrimitiveInst, RectPrimitive,
@@ -253,7 +253,7 @@ impl<'a> Painter<'a> {
         }
     }
 
-    pub fn known_len<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<Len> {
+    pub fn known_len<W: ?Sized>(&mut self, id: &StrongWidget<W>, axis: Axis) -> Option<LayoutLen> {
         let len = if let Some(len) = self.rsc.widgets().get_dyn(id.id())?.size_hint(axis) {
             Some(len.fold_dp(self.density()))
         } else if !self.reuse_child_sizes[match axis {
@@ -480,7 +480,7 @@ impl<'a> Painter<'a> {
     }
 
     /// Physical pixels per `dp` -- see `UiRenderState::density`'s field
-    /// doc. What `Len::dp`'s `apply_rest` call resolves against.
+    /// doc. What `LayoutLen::dp`'s `apply_rest` call resolves against.
     pub fn density(&self) -> f32 {
         self.render_state.density
     }

core/src/ui/render_state.rs

@@ -31,7 +31,7 @@ pub struct UiRenderState {
     pub primitives: Primitives,
     pub layers: PrimitiveLayers,
     pub(super) output_size: Vec2,
-    /// Physical pixels per `dp` -- see `Len::dp`'s field doc. `1.0` (an
+    /// Physical pixels per `dp` -- see `LayoutLen::dp`'s field doc. `1.0` (an
     /// unscaled display) until a backend that knows its own density calls
     /// `set_density` (Android's `content_scale`, read at `surface_changed`
     /// time); the winit backend has no analogous per-monitor value wired up
@@ -207,7 +207,7 @@ impl UiRenderState {
         self.resized = true;
     }
 
-    /// Sets the physical-pixels-per-dp ratio every `Len::dp` in the tree
+    /// Sets the physical-pixels-per-dp ratio every `LayoutLen::dp` in the tree
     /// resolves against from the next layout pass on -- see `density`'s
     /// field doc. Not folded into `resize` because the two change on
     /// different triggers (a surface resize on every rotation or keyboard
@@ -485,7 +485,7 @@ impl UiRenderState {
         debug_assert!(
             size.x.dp == 0.0 && size.y.dp == 0.0,
             "widget {id:?} reported an unresolved `dp` size ({size:?}); \
-             report `Len::fold_dp(painter.density())` instead"
+             report `LayoutLen::fold_dp(painter.density())` instead"
         );
         for (axis, hint) in [Axis::X, Axis::Y]
             .into_iter()

core/src/widget/mod.rs

@@ -1,4 +1,4 @@
-use crate::{Axis, Len, Painter, Size};
+use crate::{Axis, LayoutLen, Painter, Size};
 use std::any::Any;
 
 mod data;
@@ -28,7 +28,7 @@ pub enum ChildOrder {
 pub trait Widget: Any {
     fn draw(&mut self, painter: &mut Painter);
 
-    fn size_hint(&self, _axis: Axis) -> Option<Len> {
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
         None
     }
 
@@ -63,8 +63,8 @@ impl Widget for () {
         true
     }
 
-    fn size_hint(&self, _axis: Axis) -> Option<Len> {
-        Some(Len::ZERO)
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::ZERO)
     }
 }
 

src/layout_tests.rs

@@ -18,7 +18,7 @@ struct FixedRect(f32);
 
 impl Widget for FixedRect {
     fn draw(&mut self, painter: &mut Painter) {
-        let size = Size::from_axis(Axis::Y, Len::abs(self.0), Len::REST);
+        let size = Size::from_axis(Axis::Y, LayoutLen::abs(self.0), LayoutLen::REST);
         let paint = painter.paint(&PaintId::WHITE);
         painter.primitive_within(
             RectPrimitive::color(paint),
@@ -50,7 +50,11 @@ struct TracedLeaf {
 impl Widget for TracedLeaf {
     fn draw(&mut self, painter: &mut Painter) {
         self.trace.borrow_mut().push("child");
-        painter.set_size(Size::from_axis(Axis::Y, Len::abs(self.height), Len::REST));
+        painter.set_size(Size::from_axis(
+            Axis::Y,
+            LayoutLen::abs(self.height),
+            LayoutLen::REST,
+        ));
     }
 }
 
@@ -68,7 +72,11 @@ struct CountedLeaf {
 impl Widget for CountedLeaf {
     fn draw(&mut self, painter: &mut Painter) {
         self.draws.set(self.draws.get() + 1);
-        painter.set_size(Size::from_axis(Axis::Y, Len::abs(self.height), Len::REST));
+        painter.set_size(Size::from_axis(
+            Axis::Y,
+            LayoutLen::abs(self.height),
+            LayoutLen::REST,
+        ));
     }
 }
 
@@ -146,7 +154,10 @@ fn a_size_dependent_parent_is_invalidated_before_layout_runs_downward() {
     render.update(&root, &mut rsc);
 
     assert_eq!(&*trace.borrow(), &["parent", "child"]);
-    assert_eq!(render.active[&parent_weak.id()].size.y, Len::abs(40.0));
+    assert_eq!(
+        render.active[&parent_weak.id()].size.y,
+        LayoutLen::abs(40.0)
+    );
 }
 
 #[test]
@@ -192,7 +203,7 @@ fn a_span_reuses_unchanged_sibling_sizes_when_only_its_along_extent_changes() {
     let span = rsc.ui.widgets.add_strong(Span {
         children: vec![changed.any(), sibling.any()],
         dir: Dir::DOWN,
-        gap: Len::ZERO,
+        gap: LayoutLen::ZERO,
     });
     let root = rsc
         .ui
@@ -275,7 +286,7 @@ fn a_hinted_rest_draws_once_and_only_moves_the_fixed_child_after_it() {
     let fill = rsc.ui.widgets.add_strong(Sized {
         inner: fill.any(),
         x: None,
-        y: Some(Len::REST),
+        y: Some(LayoutLen::REST),
     });
     let last = rsc.ui.widgets.add_strong(FixedRect(40.0));
     let last_w = last.weak();
@@ -308,7 +319,7 @@ fn scrolled_rects(
         let row = rsc.ui.widgets.add_strong(Sized {
             inner: rect.any(),
             x: None,
-            y: Some(Len::abs(10.0)),
+            y: Some(LayoutLen::abs(10.0)),
         });
         span.push(row.any());
     }
@@ -531,7 +542,7 @@ fn a_scroll_measures_the_box_it_was_offered_not_the_window() {
     let tall = rsc.ui.widgets.add_strong(Sized {
         inner: rect.any(),
         x: None,
-        y: Some(Len::abs(1000.0)),
+        y: Some(LayoutLen::abs(1000.0)),
     });
     let scroll = rsc
         .ui
@@ -542,7 +553,7 @@ fn a_scroll_measures_the_box_it_was_offered_not_the_window() {
     let capped = rsc.ui.widgets.add_strong(MaxSize {
         inner: scroll.any(),
         x: None,
-        y: Some(Len::abs(100.0)),
+        y: Some(LayoutLen::abs(100.0)),
     });
     let capped_id = capped.id();
     let root = capped.any();
@@ -555,11 +566,11 @@ fn a_scroll_measures_the_box_it_was_offered_not_the_window() {
 
     assert_eq!(
         render.active.get(&scroll_id).unwrap().size.y,
-        Len::abs(1000.0)
+        LayoutLen::abs(1000.0)
     );
     assert_eq!(
         render.active.get(&capped_id).unwrap().size.y,
-        Len::abs(100.0),
+        LayoutLen::abs(100.0),
         "the cap, not the content and not the window"
     );
 
@@ -579,7 +590,7 @@ fn a_panned_widgets_own_hit_box_moves_exactly_once() {
     let tall = rsc.ui.widgets.add_strong(Sized {
         inner: rect.any(),
         x: None,
-        y: Some(Len::abs(1000.0)),
+        y: Some(LayoutLen::abs(1000.0)),
     });
     let tall_w = tall.weak();
     let scroll = rsc
@@ -624,7 +635,7 @@ fn a_masked_widget_keeps_one_mask_slot_that_is_always_its_own_region() {
     let capped = rsc.ui.widgets.add_strong(MaxSize {
         inner: masked.any(),
         x: None,
-        y: Some(Len::abs(60.0)),
+        y: Some(LayoutLen::abs(60.0)),
     });
     let mut span = Span::empty(Dir::DOWN);
     span.push(filler.any());
@@ -658,12 +669,12 @@ fn a_dp_cap_is_reported_in_pixels_so_a_span_can_place_it() {
     let tall = rsc.ui.widgets.add_strong(Sized {
         inner: rect.any(),
         x: None,
-        y: Some(Len::abs(1000.0)),
+        y: Some(LayoutLen::abs(1000.0)),
     });
     let capped = rsc.ui.widgets.add_strong(MaxSize {
         inner: tall.any(),
         x: None,
-        y: Some(Len::dp(100.0)),
+        y: Some(LayoutLen::dp(100.0)),
     });
     let capped_w = capped.weak();
     let filler = rsc.ui.widgets.add_strong(Rect::new(PaintId::BLACK));
@@ -698,7 +709,7 @@ fn a_size_independent_widget_moved_by_its_parent_has_the_hit_box_it_is_drawn_at(
     let spacer = rsc.ui.widgets.add_strong(Sized {
         inner: top.any(),
         x: None,
-        y: Some(Len::abs(100.0)),
+        y: Some(LayoutLen::abs(100.0)),
     });
     let spacer_w = spacer.weak();
     let below = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE));
@@ -718,7 +729,7 @@ fn a_size_independent_widget_moved_by_its_parent_has_the_hit_box_it_is_drawn_at(
         first.top_left
     );
 
-    rsc.ui.widgets.get_mut(&spacer_w).unwrap().y = Some(Len::abs(250.0));
+    rsc.ui.widgets.get_mut(&spacer_w).unwrap().y = Some(LayoutLen::abs(250.0));
     render.update(&root, &mut rsc);
     let after = render.window_region(&below_w, &rsc).unwrap();
     assert!(
@@ -763,7 +774,7 @@ fn a_widget_moved_by_its_parent_and_then_placed_inside_it_lands_at_the_placement
     let child = rsc.ui.widgets.add_strong(Sized {
         inner: rect.any(),
         x: None,
-        y: Some(Len::abs(40.0)),
+        y: Some(LayoutLen::abs(40.0)),
     });
     let child_w = child.weak();
     let parent = rsc.ui.widgets.add_strong(MoveThenPlace {
@@ -998,7 +1009,7 @@ fn a_scroll_area_opens_at_the_start_of_content_it_has_not_measured_yet() {
         let tall = rsc.ui.widgets.add_strong(Sized {
             inner: fill,
             x: None,
-            y: Some(Len::abs(5000.0)),
+            y: Some(LayoutLen::abs(5000.0)),
         });
         let scroll = rsc
             .ui
@@ -1032,7 +1043,7 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
     let header = rsc.ui.widgets.add_strong(Sized {
         inner: header_fill,
         x: None,
-        y: Some(Len::abs(HEADER)),
+        y: Some(LayoutLen::abs(HEADER)),
     });
     let mut inner = Span::empty(Dir::DOWN);
     let mut rects = Vec::new();
@@ -1042,7 +1053,7 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
         let sized = rsc.ui.widgets.add_strong(Sized {
             inner: rect.any(),
             x: None,
-            y: Some(Len::abs(ROW)),
+            y: Some(LayoutLen::abs(ROW)),
         });
         let padded = rsc.ui.widgets.add_strong(Pad {
             padding: Padding::uniform(PAD),
@@ -1056,7 +1067,7 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
         });
         let wide = rsc.ui.widgets.add_strong(Sized {
             inner: card.any(),
-            x: Some(Len::rest(1.0)),
+            x: Some(LayoutLen::rest(1.0)),
             y: None,
         });
         inner.push(wide.any());
@@ -1065,7 +1076,7 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
     let outer = rsc.ui.widgets.add_strong(Span {
         children: vec![header.any(), inner.any()],
         dir: Dir::DOWN,
-        gap: Len::ZERO,
+        gap: LayoutLen::ZERO,
     });
     let mut list = LazySpan::new(Dir::DOWN, Pin::End);
     list.push_back(LazyItem::new(0, outer.any()));
@@ -1101,6 +1112,40 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
     }
 }
 
+#[test]
+fn rest_padding_claims_leftover_space_around_fixed_content() {
+    let mut rsc = TestRsc { ui: Ui::default() };
+    let content = rect(PaintId::WHITE).sized((20, 10)).add_strong(&mut rsc);
+    let content_id = content.weak();
+    let padded = rsc.ui.widgets.add_strong(Pad {
+        inner: content.any(),
+        padding: Padding {
+            left: LayoutLen::rest(0.5),
+            right: LayoutLen::rest(0.5),
+            ..Padding::ZERO
+        },
+        exact_region: false,
+    });
+    let root = rsc
+        .ui
+        .widgets
+        .add_strong(Sized {
+            inner: padded.any(),
+            x: Some(LayoutLen::abs(100)),
+            y: None,
+        })
+        .any();
+    let mut render = UiRenderState::new();
+    render.resize((100.0, 40.0));
+
+    render.update(&root, &mut rsc);
+    render.update(&root, &mut rsc);
+
+    let region = render.window_region(&content_id, &rsc).unwrap();
+    assert_eq!(region.top_left.x, 40.0);
+    assert_eq!(region.bot_right.x, 60.0);
+}
+
 #[test]
 fn a_new_child_in_a_growing_lazy_row_uses_its_final_box_immediately() {
     const FIRST: f32 = 30.0;
@@ -1113,15 +1158,15 @@ fn a_new_child_in_a_growing_lazy_row_uses_its_final_box_immediately() {
     let first = rsc.ui.widgets.add_strong(Sized {
         inner: first.any(),
         x: None,
-        y: Some(Len::abs(FIRST)),
+        y: Some(LayoutLen::abs(FIRST)),
     });
-    let mut contents = Span::empty(Dir::DOWN).gap(Len::abs(GAP));
+    let mut contents = Span::empty(Dir::DOWN).gap(LayoutLen::abs(GAP));
     contents.push(first.any());
     let contents = rsc.ui.widgets.add_strong(contents);
     let contents_w = contents.weak();
     let row = rsc.ui.widgets.add_strong(Sized {
         inner: contents.any(),
-        x: Some(Len::rest(1.0)),
+        x: Some(LayoutLen::rest(1.0)),
         y: None,
     });
     let mut list = LazySpan::new(Dir::DOWN, Pin::End);
@@ -1137,7 +1182,7 @@ fn a_new_child_in_a_growing_lazy_row_uses_its_final_box_immediately() {
     let second = rsc.ui.widgets.add_strong(Sized {
         inner: second.any(),
         x: None,
-        y: Some(Len::abs(SECOND)),
+        y: Some(LayoutLen::abs(SECOND)),
     });
     rsc.ui
         .widgets

src/rsc/sense_tests.rs

@@ -277,7 +277,7 @@ fn a_finger_drag_over_a_scroll_area_pans_it() {
     };
 
     let scroll_strong = rect(PaintId::WHITE)
-        .height(Len::abs(1000.0))
+        .height(LayoutLen::abs(1000.0))
         .scrollable(Axis::Y, Pin::Start)
         .add_strong(&mut rsc);
     let scroll = scroll_strong.weak();
@@ -367,7 +367,7 @@ fn a_scroll_area_that_captured_the_pointer_learns_its_gesture_ended() {
         events: EventManager::default(),
     };
     let scroll_strong = rect(PaintId::WHITE)
-        .height(Len::abs(1000.0))
+        .height(LayoutLen::abs(1000.0))
         .scrollable(Axis::Y, Pin::Start)
         .add_strong(&mut rsc);
     let scroll = scroll_strong.weak();
@@ -522,8 +522,8 @@ fn a_drag_pans_whichever_nested_scroll_area_owns_its_axis() {
         let seen = Rc::new(Cell::new(None));
         let record = seen.clone();
         let outer_strong = rect(PaintId::WHITE)
-            .width(Len::abs(1000.0))
-            .height(Len::abs(1000.0))
+            .width(LayoutLen::abs(1000.0))
+            .height(LayoutLen::abs(1000.0))
             .scrollable(Axis::X, Pin::Start)
             .with_id(move |_rsc, id| {
                 record.set(Some(id));
@@ -581,13 +581,13 @@ fn a_press_does_not_reach_a_widget_the_pointer_has_just_left() {
     let half = |slot: &Rc<Cell<Option<WeakWidget<ScrollArea>>>>| {
         let record = slot.clone();
         rect(PaintId::WHITE)
-            .height(Len::abs(1000.0))
+            .height(LayoutLen::abs(1000.0))
             .scrollable(Axis::Y, Pin::Start)
             .with_id(move |_rsc, id| {
                 record.set(Some(id));
                 id
             })
-            .height(Len::rel(0.5))
+            .height(LayoutLen::rel(0.5))
     };
     let root = (half(&seen[0]), half(&seen[1]))
         .span(Dir::DOWN)

src/widget/image.rs

@@ -12,8 +12,8 @@ impl Widget for Image {
         painter.set_size(Size::abs(size));
     }
 
-    fn size_hint(&self, axis: Axis) -> Option<Len> {
-        Some(Len::abs(self.handle.size().axis(axis)))
+    fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::abs(self.handle.size().axis(axis)))
     }
 
     fn is_size_independent(&self) -> bool {

src/widget/layout/lazy_span.rs

@@ -811,8 +811,8 @@ impl Widget for LazySpan {
         painter.set_size(Size::REST);
     }
 
-    fn size_hint(&self, _axis: Axis) -> Option<Len> {
-        Some(Len::REST)
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::REST)
     }
 }
 
@@ -850,7 +850,7 @@ mod tests {
         let sized = rsc.ui.widgets.add_strong(Sized {
             inner: rect.any(),
             x: None,
-            y: Some(Len::abs(height)),
+            y: Some(LayoutLen::abs(height)),
         });
         (sized.weak(), sized.any())
     }
@@ -1054,7 +1054,7 @@ mod tests {
         let fg = rsc.ui.widgets.add_strong(Sized {
             inner: fg_rect.any(),
             x: None,
-            y: Some(Len::abs(height)),
+            y: Some(LayoutLen::abs(height)),
         });
         let fg_weak = fg.weak();
         let stack = Stack {
@@ -1084,7 +1084,7 @@ mod tests {
                 render.update(&root, &mut rsc);
 
                 let (bg_id, fg) = rows[key_to_change as usize];
-                rsc.ui.widgets.get_mut(&fg).unwrap().y = Some(Len::abs(new_height));
+                rsc.ui.widgets.get_mut(&fg).unwrap().y = Some(LayoutLen::abs(new_height));
                 render.update(&root, &mut rsc);
 
                 let px = render
@@ -1190,7 +1190,7 @@ mod tests {
         }
 
         rsc.ui.widgets.get_mut(&list_weak).unwrap().note_tap(41.0);
-        rsc.ui.widgets.get_mut(&row2).unwrap().y = Some(Len::abs(50.0));
+        rsc.ui.widgets.get_mut(&row2).unwrap().y = Some(LayoutLen::abs(50.0));
 
         render.update(&root, &mut rsc);
 
@@ -1227,7 +1227,7 @@ mod tests {
 
         let row2 = rows[2];
         rsc.ui.widgets.get_mut(&list_weak).unwrap().note_tap(59.0);
-        rsc.ui.widgets.get_mut(&row2).unwrap().y = Some(Len::abs(50.0));
+        rsc.ui.widgets.get_mut(&row2).unwrap().y = Some(LayoutLen::abs(50.0));
         render.update(&root, &mut rsc);
 
         let list_ref = rsc.ui.widgets.get(&list_weak).unwrap();

src/widget/layout/max_size.rs

@@ -2,12 +2,12 @@ use crate::prelude::*;
 
 pub struct MaxSize {
     pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
+    pub x: Option<LayoutLen>,
+    pub y: Option<LayoutLen>,
 }
 
 impl MaxSize {
-    fn clamp(len: Len, max: Option<Len>, output: f32, density: f32) -> Len {
+    fn clamp(len: LayoutLen, max: Option<LayoutLen>, output: f32, density: f32) -> LayoutLen {
         let Some(max) = max else {
             return len;
         };
@@ -20,7 +20,7 @@ impl MaxSize {
         }
     }
 
-    fn clamp_region(offered_px: f32, max: Option<Len>, output: f32, density: f32) -> UiSpan {
+    fn clamp_region(offered_px: f32, max: Option<LayoutLen>, output: f32, density: f32) -> UiSpan {
         let Some(max) = max else {
             return UiSpan::FULL;
         };

src/widget/layout/pad.rs

@@ -10,16 +10,14 @@ impl Widget for Pad {
     fn draw(&mut self, painter: &mut Painter) {
         let density = painter.density();
         let offered = painter.px_size();
-        let region = self.padding.region(density);
-        let used = painter.widget_within(&self.inner, region).size();
+        let used = painter
+            .widget_within(&self.inner, self.padding.fixed_region(density))
+            .size();
+        let region = self.padding.region_for(used, density);
         painter.place_used(&self.inner, used, region);
-        let width =
-            self.padding.left.apply_rest(density).abs + self.padding.right.apply_rest(density).abs;
-        let height =
-            self.padding.top.apply_rest(density).abs + self.padding.bottom.apply_rest(density).abs;
         let size = Size {
-            x: used.x + Len::abs(width),
-            y: used.y + Len::abs(height),
+            x: self.padding.left.fold_dp(density) + used.x + self.padding.right.fold_dp(density),
+            y: self.padding.top.fold_dp(density) + used.y + self.padding.bottom.fold_dp(density),
         };
         let needed = size.to_uivec2(density).to_abs(painter.output_size());
         if needed.x <= offered.x + 0.01 && needed.y <= offered.y + 0.01 {
@@ -34,21 +32,21 @@ impl Widget for Pad {
 }
 
 pub struct Padding {
-    pub left: Len,
-    pub right: Len,
-    pub top: Len,
-    pub bottom: Len,
+    pub left: LayoutLen,
+    pub right: LayoutLen,
+    pub top: LayoutLen,
+    pub bottom: LayoutLen,
 }
 
 impl Padding {
     pub const ZERO: Self = Self {
-        left: Len::ZERO,
-        right: Len::ZERO,
-        top: Len::ZERO,
-        bottom: Len::ZERO,
+        left: LayoutLen::ZERO,
+        right: LayoutLen::ZERO,
+        top: LayoutLen::ZERO,
+        bottom: LayoutLen::ZERO,
     };
 
-    pub fn uniform(amt: impl Into<Len>) -> Self {
+    pub fn uniform(amt: impl Into<LayoutLen>) -> Self {
         let amt = amt.into();
         Self {
             left: amt,
@@ -57,79 +55,119 @@ impl Padding {
             bottom: amt,
         }
     }
-    pub fn region(&self, density: f32) -> UiRegion {
+    fn fixed_region(&self, density: f32) -> UiRegion {
         let mut region = UiRegion::FULL;
-        region.x.start.abs += self.left.apply_rest(density).abs;
-        region.y.start.abs += self.top.apply_rest(density).abs;
-        region.x.end.abs -= self.right.apply_rest(density).abs;
-        region.y.end.abs -= self.bottom.apply_rest(density).abs;
+        let left = self.left.fold_dp(density);
+        let right = self.right.fold_dp(density);
+        let top = self.top.fold_dp(density);
+        let bottom = self.bottom.fold_dp(density);
+        region.x.start += UiScalar::new(left.rel, left.abs);
+        region.x.end -= UiScalar::new(right.rel, right.abs);
+        region.y.start += UiScalar::new(top.rel, top.abs);
+        region.y.end -= UiScalar::new(bottom.rel, bottom.abs);
         region
     }
-    pub fn x(amt: impl Into<Len>) -> Self {
+
+    fn region_for(&self, inner: Size, density: f32) -> UiRegion {
+        UiRegion {
+            x: padded_axis(self.left, inner.x, self.right, density),
+            y: padded_axis(self.top, inner.y, self.bottom, density),
+        }
+    }
+    pub fn x(amt: impl Into<LayoutLen>) -> Self {
         let amt = amt.into();
         Self {
             left: amt,
             right: amt,
-            top: Len::ZERO,
-            bottom: Len::ZERO,
+            top: LayoutLen::ZERO,
+            bottom: LayoutLen::ZERO,
         }
     }
-    pub fn y(amt: impl Into<Len>) -> Self {
+    pub fn y(amt: impl Into<LayoutLen>) -> Self {
         let amt = amt.into();
         Self {
-            left: Len::ZERO,
-            right: Len::ZERO,
+            left: LayoutLen::ZERO,
+            right: LayoutLen::ZERO,
             top: amt,
             bottom: amt,
         }
     }
 
-    pub fn top(amt: impl Into<Len>) -> Self {
+    pub fn top(amt: impl Into<LayoutLen>) -> Self {
         let mut s = Self::ZERO;
         s.top = amt.into();
         s
     }
 
-    pub fn bottom(amt: impl Into<Len>) -> Self {
+    pub fn bottom(amt: impl Into<LayoutLen>) -> Self {
         let mut s = Self::ZERO;
         s.bottom = amt.into();
         s
     }
 
-    pub fn left(amt: impl Into<Len>) -> Self {
+    pub fn left(amt: impl Into<LayoutLen>) -> Self {
         let mut s = Self::ZERO;
         s.left = amt.into();
         s
     }
 
-    pub fn right(amt: impl Into<Len>) -> Self {
+    pub fn right(amt: impl Into<LayoutLen>) -> Self {
         let mut s = Self::ZERO;
         s.right = amt.into();
         s
     }
 
-    pub fn with_top(mut self, amt: impl Into<Len>) -> Self {
+    pub fn with_top(mut self, amt: impl Into<LayoutLen>) -> Self {
         self.top = amt.into();
         self
     }
 
-    pub fn with_bottom(mut self, amt: impl Into<Len>) -> Self {
+    pub fn with_bottom(mut self, amt: impl Into<LayoutLen>) -> Self {
         self.bottom = amt.into();
         self
     }
 
-    pub fn with_left(mut self, amt: impl Into<Len>) -> Self {
+    pub fn with_left(mut self, amt: impl Into<LayoutLen>) -> Self {
         self.left = amt.into();
         self
     }
 
-    pub fn with_right(mut self, amt: impl Into<Len>) -> Self {
+    pub fn with_right(mut self, amt: impl Into<LayoutLen>) -> Self {
         self.right = amt.into();
         self
     }
 }
 
-impl<T: Into<Len>> From<T> for Padding {
+fn padded_axis(before: LayoutLen, inner: LayoutLen, after: LayoutLen, density: f32) -> UiSpan {
+    let lengths = [
+        before.fold_dp(density),
+        inner.fold_dp(density),
+        after.fold_dp(density),
+    ];
+    let total = lengths
+        .iter()
+        .copied()
+        .fold(LayoutLen::ZERO, |sum, len| sum + len);
+    let fixed = UiScalar::new(total.rel, total.abs);
+    let mut cursor = UiScalar::ZERO;
+    let mut inner_span = UiSpan::FULL;
+    for (index, len) in lengths.into_iter().enumerate() {
+        let start = cursor;
+        if len.rest > 0.0 {
+            let rest_end = UiScalar::rel(len.rest / total.rest);
+            let available_end = (UiScalar::FULL + cursor) - fixed;
+            cursor = rest_end.within(&cursor.to(available_end));
+        }
+        cursor.rel += len.rel;
+        cursor.abs += len.abs;
+        if index == 1 {
+            inner_span = start.to(cursor);
+        }
+    }
+    inner_span
+}
+
+impl<T: Into<LayoutLen>> From<T> for Padding {
     fn from(amt: T) -> Self {
         Self::uniform(amt.into())
     }

src/widget/layout/scroll_area.rs

@@ -97,7 +97,7 @@ mod tests {
         let mut rsc = TestRsc { ui: Ui::default() };
         let fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE)).any();
         let id = fill.id();
-        let long = Some(Len::abs(1000.0));
+        let long = Some(LayoutLen::abs(1000.0));
         let tall = rsc.ui.widgets.add_strong(Sized {
             inner: fill,
             x: (axis == Axis::X).then_some(long).flatten(),

src/widget/layout/sized.rs

@@ -2,8 +2,8 @@ use crate::prelude::*;
 
 pub struct Sized {
     pub inner: StrongWidget,
-    pub x: Option<Len>,
-    pub y: Option<Len>,
+    pub x: Option<LayoutLen>,
+    pub y: Option<LayoutLen>,
 }
 
 impl Widget for Sized {
@@ -25,7 +25,7 @@ impl Widget for Sized {
         painter.set_size(size);
     }
 
-    fn size_hint(&self, axis: Axis) -> Option<Len> {
+    fn size_hint(&self, axis: Axis) -> Option<LayoutLen> {
         match axis {
             Axis::X => self.x,
             Axis::Y => self.y,

src/widget/layout/span.rs

@@ -4,7 +4,7 @@ use std::marker::PhantomData;
 pub struct Span {
     pub children: Vec<StrongWidget>,
     pub dir: Dir,
-    pub gap: Len,
+    pub gap: LayoutLen,
 }
 
 impl Widget for Span {
@@ -16,7 +16,7 @@ impl Widget for Span {
         let axis = self.dir.axis;
         let gap = self.gap.apply_rest(painter.density()).abs;
 
-        let mut lens: Vec<Option<Len>> = self
+        let mut lens: Vec<Option<LayoutLen>> = self
             .children
             .iter()
             .map(|child| painter.known_len(child, axis))
@@ -43,13 +43,13 @@ impl Widget for Span {
             cursor.rel += len.rel;
         }
 
-        let lens: Vec<Len> = lens.into_iter().map(Option::unwrap).collect();
+        let lens: Vec<LayoutLen> = lens.into_iter().map(Option::unwrap).collect();
 
         let gap_total = gap * self.children.len().saturating_sub(1) as f32;
-        let total = lens.iter().fold(Len::abs(gap_total), |s, &l| s + l);
+        let total = lens.iter().fold(LayoutLen::abs(gap_total), |s, &l| s + l);
 
         let mut start = UiScalar::rel_min();
-        let mut ortho_len = Len::ZERO;
+        let mut ortho_len = LayoutLen::ZERO;
         let mut ortho_mixed = false;
         let mut placed = Vec::with_capacity(self.children.len());
         for (i, (child, &len)) in self.children.iter().zip(&lens).enumerate() {
@@ -84,7 +84,7 @@ impl Widget for Span {
             }
         }
         if ortho_mixed {
-            ortho_len = Len::default();
+            ortho_len = LayoutLen::default();
         } else {
             let ortho = ortho_len
                 .apply_rest(painter.density())
@@ -98,7 +98,7 @@ impl Widget for Span {
         let along = if total.rest == 0.0 && total.rel == 0.0 {
             total
         } else {
-            Len::default()
+            LayoutLen::default()
         };
 
         painter.set_size(Size::from_axis(axis, along, ortho_len));
@@ -110,11 +110,11 @@ impl Span {
         Self {
             children: Vec::new(),
             dir,
-            gap: Len::ZERO,
+            gap: LayoutLen::ZERO,
         }
     }
 
-    pub fn gap(mut self, gap: impl Into<Len>) -> Self {
+    pub fn gap(mut self, gap: impl Into<LayoutLen>) -> Self {
         self.gap = gap.into();
         self
     }
@@ -131,7 +131,7 @@ impl Span {
 pub struct SpanBuilder<State, const LEN: usize, Wa: WidgetArrLike<State, LEN, Tag>, Tag> {
     pub children: Wa,
     pub dir: Dir,
-    pub gap: Len,
+    pub gap: LayoutLen,
     _pd: PhantomData<(State, Tag)>,
 }
 
@@ -157,12 +157,12 @@ impl<State, const LEN: usize, Wa: WidgetArrLike<State, LEN, Tag>, Tag>
         Self {
             children,
             dir,
-            gap: Len::ZERO,
+            gap: LayoutLen::ZERO,
             _pd: PhantomData,
         }
     }
 
-    pub fn gap(mut self, gap: impl Into<Len>) -> Self {
+    pub fn gap(mut self, gap: impl Into<LayoutLen>) -> Self {
         self.gap = gap.into();
         self
     }

src/widget/layout/stack.rs

@@ -53,9 +53,9 @@ impl Widget for Stack {
         painter.set_size(size);
     }
 
-    fn size_hint(&self, _axis: Axis) -> Option<Len> {
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
         match self.size {
-            StackSize::Default => Some(Len::REST),
+            StackSize::Default => Some(LayoutLen::REST),
             StackSize::Child(_) => None,
         }
     }

src/widget/rect.rs

@@ -49,8 +49,8 @@ impl Widget for Rect {
         painter.set_size(Size::REST);
     }
 
-    fn size_hint(&self, _axis: Axis) -> Option<Len> {
-        Some(Len::REST)
+    fn size_hint(&self, _axis: Axis) -> Option<LayoutLen> {
+        Some(LayoutLen::REST)
     }
 }
 

src/widget/trait_fns.rs

@@ -41,7 +41,7 @@ widget_trait! {
         }
     }
 
-    fn max_width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+    fn max_width(self, len: impl Into<LayoutLen>) -> impl WidgetFn<Rsc, MaxSize> {
         let len = len.into();
         move |state| MaxSize {
             inner: self.add_strong(state),
@@ -50,7 +50,7 @@ widget_trait! {
         }
     }
 
-    fn max_height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, MaxSize> {
+    fn max_height(self, len: impl Into<LayoutLen>) -> impl WidgetFn<Rsc, MaxSize> {
         let len = len.into();
         move |state| MaxSize {
             inner: self.add_strong(state),
@@ -59,7 +59,7 @@ widget_trait! {
         }
     }
 
-    fn width(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
+    fn width(self, len: impl Into<LayoutLen>) -> impl WidgetFn<Rsc, Sized> {
         let len = len.into();
         move |state| Sized {
             inner: self.add_strong(state),
@@ -68,7 +68,7 @@ widget_trait! {
         }
     }
 
-    fn height(self, len: impl Into<Len>) -> impl WidgetFn<Rsc, Sized> {
+    fn height(self, len: impl Into<LayoutLen>) -> impl WidgetFn<Rsc, Sized> {
         let len = len.into();
         move |state| Sized {
             inner: self.add_strong(state),