Retain child frames relative to the container extent

core/src/ui/active.rs

@@ -41,7 +41,7 @@ pub struct ActiveData {
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<RetainedPrimitive>,
     pub mask_region: Option<DrawRegion>,
-    pub inherited_children: Vec<WidgetId>,
+    pub(crate) extent_children: Vec<(WidgetId, ExtentPlacement)>,
     pub children: Vec<WidgetId>,
     /// The children whose size this widget read while drawing.
     pub size_deps: Vec<WidgetId>,
@@ -87,3 +87,18 @@ impl ActiveData {
         })
     }
 }
+
+#[derive(Clone, Copy, Debug)]
+pub(crate) enum ExtentPlacement {
+    Inherit,
+    Within(UiRegion),
+}
+
+impl ExtentPlacement {
+    pub fn resolve(self, extent: UiRegion) -> (UiRegion, [Option<crate::UiSpan>; 2]) {
+        match self {
+            Self::Inherit => (UiRegion::FULL, [Some(extent.x), Some(extent.y)]),
+            Self::Within(local) => (local.within(&extent), [None; 2]),
+        }
+    }
+}

core/src/ui/painter.rs

@@ -1,9 +1,9 @@
 #[cfg(feature = "layout-diagnostics")]
 use crate::layout_diagnostics::{self as diag, Counter};
 use crate::{
-    Axis, DrawRegion, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign, RenderedText,
+    Axis, DrawRegion, ExtentPlacement, Holds, LayoutLen, Len, Px, PxVec2, RegionAlign,
-    RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData, TextureHandle,
+    RenderedText, RetainedPrimitive, Size, StrongWidget, TextAttrs, TextBuffer, TextData,
-    UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
+    TextureHandle, UiRegion, UiRenderState, UiRsc, UiSpan, UiVec2, Weight, WidgetId, Widgets,
     render::{
         GlyphPrimitive, Mask, MaskIdx, MoveIdx, Primitive, PrimitiveInst, PrimitiveKind,
         TexturePrimitive,
@@ -40,7 +40,7 @@ pub struct Painter<'a> {
     pub(super) textures: Vec<TextureHandle>,
     pub(super) primitives: Vec<RetainedPrimitive>,
     pub(super) mask_region: Option<DrawRegion>,
-    pub(super) inherited_children: Vec<WidgetId>,
+    pub(super) extent_children: Vec<(WidgetId, ExtentPlacement)>,
     pub(super) extent_own: [Holds; 2],
     pub(super) extent_under: [Holds; 2],
     pub(super) children: Vec<WidgetId>,
@@ -154,7 +154,13 @@ impl<'a> Painter<'a> {
     /// around one child wants, since its box is the child's.
     pub fn widget<'s, W: ?Sized>(&'s mut self, id: &'s StrongWidget<W>) -> DrawResult<'s, 'a, W> {
         let own = self.placement;
-        self.widget_at_inner(id, UiRegion::FULL, [Some(own.x), Some(own.y)], true, false)
+        self.widget_at_inner(
+            id,
+            UiRegion::FULL,
+            [Some(own.x), Some(own.y)],
+            Some(ExtentPlacement::Inherit),
+            false,
+        )
     }
 
     /// What a widget's rules declare its lengths to be, which whoever draws
@@ -170,20 +176,29 @@ impl<'a> Painter<'a> {
     /// this frame; what it answered is still something this widget asked.
     pub fn undraw<W: ?Sized>(&mut self, id: &StrongWidget<W>) {
         self.children.retain(|child| *child != id.id());
-        self.inherited_children.retain(|child| *child != id.id());
+        self.extent_children.retain(|(child, _)| *child != id.id());
         self.state.undraw_rec(id.id(), self.rsc);
     }
 
-    /// Draws a widget somewhere within this one. `region` is in this widget's
+    /// Draws a child in a frame relative to this widget's frame or extent.
-    /// own region, and is the child's own region: what its declared lengths
+    /// A plain `UiRegion` is frame-relative. `DrawRegion::Extent` keeps the
-    /// and its report are fractions of. Where its drawing sits inside that is
+    /// child's frame attached to the extent without reading `placement()`.
-    /// its own answer placed by its alignment.
+    /// The child places its answer within that frame by its own alignment.
     pub fn widget_within<'s, W: ?Sized>(
         &'s mut self,
         id: &'s StrongWidget<W>,
-        region: UiRegion,
+        region: impl Into<DrawRegion>,
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at(id, region, [None; 2])
+        match region.into() {
+            DrawRegion::Frame(region) => self.widget_at(id, region, [None; 2]),
+            DrawRegion::Extent(local) => self.widget_at_inner(
+                id,
+                local.within(&self.placement),
+                [None; 2],
+                Some(ExtentPlacement::Within(local)),
+                false,
+            ),
+        }
     }
 
     /// Draws a widget in `region`, saying where in it the drawing goes.
@@ -206,7 +221,7 @@ impl<'a> Painter<'a> {
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
     ) -> DrawResult<'s, 'a, W> {
-        self.widget_at_inner(id, region, placement, false, false)
+        self.widget_at_inner(id, region, placement, None, false)
     }
 
     fn widget_at_inner<'s, W: ?Sized>(
@@ -214,15 +229,12 @@ impl<'a> Painter<'a> {
         id: &'s StrongWidget<W>,
         region: UiRegion,
         placement: [Option<UiSpan>; 2],
-        inherited: bool,
+        extent: Option<ExtentPlacement>,
         measuring: bool,
     ) -> DrawResult<'s, 'a, W> {
-        if inherited {
+        self.extent_children.retain(|(child, _)| *child != id.id());
-            if !self.inherited_children.contains(&id.id()) {
+        if let Some(extent) = extent {
-                self.inherited_children.push(id.id());
+            self.extent_children.push((id.id(), extent));
-            }
-        } else {
-            self.inherited_children.retain(|child| *child != id.id());
         }
         let region_node = self.rsc.widgets().is_region_node(id.id());
         let declared = self.declared_lens(id);
@@ -285,28 +297,53 @@ impl<'a> Painter<'a> {
             measuring,
             self.rsc,
         );
-        // Whatever the child's answer holds for keeps this one to the boxes
-        // that give the child a length inside it.
         for axis in AXES {
             let n = axis as usize;
-            let frame = holds.frame[n].through(local.axis(axis).len());
+            match extent {
-            self.under[n] = self.under[n].and(frame);
+                Some(ExtentPlacement::Inherit) if declared[n].is_none() => {
-            if inherited && declared[n].is_none() {
+                    self.under[n] =
+                        self.under[n].and(holds.frame[n].through(local.axis(axis).len()));
                     self.extent_under[n] = self.extent_under[n].and(holds.extent[n]);
                     self.reads_placement |= holds.placement.is_some();
-            } else {
+                }
-                let extent = placement[n].unwrap_or(UiSpan::FULL).len();
+                Some(ExtentPlacement::Within(part))
-                self.under[n] = self.under[n].and(
+                    if declared[n].is_none()
+                        && part.axis(axis).start.rel == crate::Rel::ZERO
+                        && part.axis(axis).end.rel == crate::Rel::ONE =>
+                {
+                    let dependent = holds.frame[n]
+                        .and(holds.extent[n])
+                        .through(part.axis(axis).len());
+                    self.extent_under[n] = self.extent_under[n].and(dependent);
+                }
+                _ => {
+                    let chosen = placement[n].unwrap_or(UiSpan::FULL).len();
+                    self.under[n] = self.under[n]
+                        .and(holds.frame[n].through(local.axis(axis).len()))
+                        .and(
                             holds.extent[n]
-                        .through(extent)
+                                .through(chosen)
                                 .through(local.axis(axis).len()),
                         );
+                    // Fractional endpoints compose before pixel evaluation. Their
+                    // difference cannot be inverted through the extent length alone.
+                    if matches!(extent, Some(ExtentPlacement::Within(_))) && declared[n].is_none() {
+                        self.reads_placement = true;
                     }
                 }
+            }
+        }
+        // A fractional report is composed into the parent frame, so its
+        // value can change with the extent even when the drawing holds.
+        let reads_placement = matches!(extent, Some(ExtentPlacement::Within(_)))
+            && AXES.into_iter().any(|axis| {
+                declared[axis as usize].is_none() && size.axis(axis).rel != crate::Rel::ZERO
+            });
         DrawResult {
             child: id,
             painter: self,
             size: in_parent_frame(size, local.size(), declared),
+            reads_placement,
         }
     }
 
@@ -363,7 +400,7 @@ impl<'a> Painter<'a> {
                 .retained_size(child.id(), px, placement, self.move_idx, self.rsc.widgets());
         let Some((size, holds)) = retained else {
             return self
-                .widget_at_inner(child, region, offered, false, true)
+                .widget_at_inner(child, region, offered, None, true)
                 .len(axis);
         };
         #[cfg(feature = "layout-diagnostics")]
@@ -604,6 +641,7 @@ pub struct DrawResult<'p, 'a, W: ?Sized> {
     painter: &'p mut Painter<'a>,
     child: &'p StrongWidget<W>,
     size: Size,
+    reads_placement: bool,
 }
 
 impl<W: ?Sized> DrawResult<'_, '_, W> {
@@ -614,6 +652,7 @@ impl<W: ?Sized> DrawResult<'_, '_, W> {
             diag::size_read(self.child.id(), self.painter.id, self.size);
         }
         self.painter.depend_on(self.child);
+        self.painter.reads_placement |= self.reads_placement;
         self.size
     }
 

core/src/ui/render_state.rs

@@ -300,7 +300,7 @@ impl UiRenderState {
             && let Some(old_parent) = self.active.get_mut(&old_parent)
         {
             old_parent.children.retain(|child| *child != id);
-            old_parent.inherited_children.retain(|child| *child != id);
+            old_parent.extent_children.retain(|(child, _)| *child != id);
         }
         settled
     }
@@ -373,7 +373,7 @@ impl UiRenderState {
             textures: Vec::new(),
             primitives: Vec::new(),
             mask_region: None,
-            inherited_children: Vec::new(),
+            extent_children: Vec::new(),
             children: Vec::new(),
             offered: Vec::new(),
             offered_px: info.offered_px,
@@ -410,7 +410,7 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            inherited_children,
+            extent_children,
             extent_own,
             extent_under,
             children,
@@ -516,7 +516,7 @@ impl UiRenderState {
             textures,
             primitives,
             mask_region,
-            inherited_children,
+            extent_children,
             children,
             size_deps,
             declared: declared_lens(rsc.widgets(), id),
@@ -765,19 +765,18 @@ impl UiRenderState {
         }
         let parent_move = active.move_idx;
         let mask = active.mask;
-        let children = active.inherited_children.len();
+        let children = active.extent_children.len();
         for index in 0..children {
-            let child = self.active[&id].inherited_children[index];
+            let (child, extent) = self.active[&id].extent_children[index];
+            let (child_region, chosen) = extent.resolve(placement);
             let active = &self.active[&child];
-            let (child_local, chosen) = ask_box(
+            let (child_local, chosen) =
-                UiRegion::FULL,
+                ask_box(child_region, active.declared, active.own_align, chosen);
-                active.declared,
+            // Keep the slot chosen from measurement: the final draw can
-                active.own_align,
+            // report a different size, for example after text reflows.
-                [Some(placement.x), Some(placement.y)],
-            );
             let child_placement = UiRegion {
-                x: chosen[0].unwrap_or(UiSpan::FULL),
+                x: chosen[0].unwrap_or(active.placement.x),
-                y: chosen[1].unwrap_or(UiSpan::FULL),
+                y: chosen[1].unwrap_or(active.placement.y),
             };
             let child_info = DrawInfo {
                 layer: active.layer,
@@ -937,7 +936,7 @@ impl UiRenderState {
                 textures: Vec::new(),
                 primitives: Vec::new(),
                 mask_region: None,
-                inherited_children: Vec::new(),
+                extent_children: Vec::new(),
                 children: Vec::new(),
                 size_deps: Vec::new(),
                 move_idx: info.parent_move,

src/widget/position/pad.rs

@@ -13,7 +13,7 @@ impl Widget for Pad {
         // it; where the box is bigger -- a share of a row, a rule over this
         // widget -- the slack is the inner's to sit in, and forcing the near
         // edge pinned it to a corner it had not asked for.
-        let inside = self.padding.region_of(painter.placement());
+        let inside = DrawRegion::Extent(self.padding.region());
         let inner = painter.widget_within(&self.inner, inside).size();
         Size {
             x: LayoutLen {

src/widget/position/stack.rs

@@ -13,8 +13,6 @@ impl Widget for Stack {
             StackSize::Default => None,
             StackSize::Child(i) => Some(i),
         };
-        // This stack's own box, which is `FULL` until its answer is known.
-        let placement = painter.placement();
         // Whichever child sizes the stack keeps the stack's whole region as
         // its own -- the stack is the length that child asked for, so taking
         // the fraction of the stack's box again would take it twice -- and is
@@ -25,13 +23,7 @@ impl Widget for Stack {
             // drawing belongs to the layer it was made on.
             Some((i, child)) => {
                 painter.child_layer_at(i);
-                painter
+                painter.widget(child).size()
-                    .widget_at(
-                        child,
-                        UiRegion::FULL,
-                        [Some(placement.x), Some(placement.y)],
-                    )
-                    .size()
             }
             None => Size::LEFTOVER,
         };
@@ -43,7 +35,7 @@ impl Widget for Stack {
             // Every other child has the stack's own box for its region, since
             // the stack is what contains it, and where it sits in one bigger
             // than itself is its own business.
-            painter.widget_within(child, placement);
+            painter.widget_within(child, DrawRegion::Extent(UiRegion::FULL));
         }
         size
     }

tests/cases/retained.rs

@@ -1020,3 +1020,212 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         }
     }
 }
+
+#[test]
+fn padding_and_stack_frames_follow_the_extent_without_drawing_again() {
+    struct Observed<W> {
+        widget: W,
+        draws: Rc<Cell<usize>>,
+    }
+    impl<W: Widget> Widget for Observed<W> {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.draws.set(self.draws.get() + 1);
+            self.widget.draw(painter)
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        extent: UiRegion,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                UiRegion::FULL,
+                [Some(self.extent.x), Some(self.extent.y)],
+            );
+            Size::LEFTOVER
+        }
+    }
+    for node in [false, true] {
+        let plant = |h: &mut Harness, extent| {
+            let draws = Rc::new(Cell::new(0));
+            let leaf = rect(Color::BLUE).masked().add(&mut h.rsc);
+            h.rsc.widgets_mut().set_region_node(leaf, node);
+            let fixed = rect(Color::RED).width(31).height(19).add(&mut h.rsc);
+            let stack = Observed {
+                widget: Stack {
+                    children: vec![leaf.add_strong(&mut h.rsc), fixed.add_strong(&mut h.rsc)],
+                    size: StackSize::Default,
+                },
+                draws: draws.clone(),
+            }
+            .add_strong(&mut h.rsc);
+            let pad = Observed {
+                widget: Pad {
+                    inner: stack,
+                    padding: Padding::uniform(7).with_left(13),
+                },
+                draws: draws.clone(),
+            }
+            .add_strong(&mut h.rsc);
+            let root = Frame { child: pad, extent }.add(&mut h.rsc);
+            h.set_root(root);
+            (root, leaf, fixed, draws)
+        };
+        let mut warm = Harness::new((403, 211));
+        let (root, leaf, fixed, draws) = plant(&mut warm, UiRegion::FULL);
+        for (start, end) in [(0.13, 0.83), (-0.17, 1.23), (0.31, 0.67)] {
+            let extent = UiRegion::new(
+                UiSpan::new(Len::rel(start) + Len::px(3.125), Len::rel(end)),
+                UiSpan::new(Len::px(11.25), Len::rel(end)),
+            );
+            let before = draws.get();
+            warm.rsc[root].extent = extent;
+            warm.frame();
+            assert_eq!(draws.get(), before);
+            let mut cold = Harness::new((403, 211));
+            let (_, other, other_fixed, _) = plant(&mut cold, extent);
+            for (a, b) in [(leaf.id(), other.id()), (fixed.id(), other_fixed.id())] {
+                assert_eq!(warm.region(&a), cold.region(&b));
+                assert_eq!(primitive_bounds(&warm, a), primitive_bounds(&cold, b));
+            }
+            let mask = |h: &Harness, id: WidgetId| {
+                let active = &h.render.active[&id];
+                let mask = &h.rsc.ui().masks[active.mask.idx()];
+                h.render
+                    .moves
+                    .resolve(mask.move_idx, mask.region)
+                    .to_px(h.render.output_size())
+            };
+            assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
+        }
+    }
+}
+
+#[test]
+fn moving_an_extent_child_preserves_the_slot_chosen_from_its_measurement() {
+    struct Measured;
+    impl Widget for Measured {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            let width = painter.px_len(Axis::X);
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((80, if width > Px::from_int(100) { 40 } else { 60 }))
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        start: f32,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter.widget_at(
+                &self.child,
+                UiRegion::FULL,
+                [
+                    Some(UiSpan::new(
+                        Len::px(self.start),
+                        Len::px(self.start + 200.0),
+                    )),
+                    Some(UiSpan::FULL),
+                ],
+            );
+            Size::LEFTOVER
+        }
+    }
+    let mut h = Harness::new((400, 200));
+    let leaf = Measured.add(&mut h.rsc);
+    let stack = (leaf,).stack().add_strong(&mut h.rsc);
+    let root = Frame {
+        child: stack,
+        start: 0.0,
+    }
+    .add(&mut h.rsc);
+    h.set_root(root);
+    assert_corners!(h, leaf, (60, 80), (140, 120));
+    h.rsc[root].start = 30.0;
+    h.frame();
+    assert_corners!(h, leaf, (90, 80), (170, 120));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf).unwrap()]
+    );
+}
+
+#[test]
+fn extent_frames_keep_fractional_reports_and_numeric_dependencies_valid() {
+    struct Container {
+        child: StrongWidget,
+        region: UiRegion,
+    }
+    impl Widget for Container {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            painter
+                .widget_within(&self.child, DrawRegion::Extent(self.region))
+                .size()
+        }
+    }
+    struct Frame {
+        child: StrongWidget,
+        extent: UiRegion,
+        answer: Rc<Cell<Size>>,
+    }
+    impl Widget for Frame {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.answer.set(
+                painter
+                    .widget_at(
+                        &self.child,
+                        UiRegion::FULL,
+                        [Some(self.extent.x), Some(self.extent.y)],
+                    )
+                    .size(),
+            );
+            Size::LEFTOVER
+        }
+    }
+    for fractional in [false, true] {
+        for region in [
+            UiRegion::FULL,
+            UiRegion::new(UiSpan::new(Len::rel(0.13), Len::rel(0.79)), UiSpan::FULL),
+        ] {
+            let plant = |h: &mut Harness, extent| {
+                let size = if fractional {
+                    Size {
+                        x: rel(0.5),
+                        y: LayoutLen::px(27),
+                    }
+                } else {
+                    Size::from((80, 27))
+                };
+                let (leaf, _) = counted(h, size, !fractional);
+                let child = Container {
+                    child: leaf.add_strong(&mut h.rsc),
+                    region,
+                }
+                .add_strong(&mut h.rsc);
+                let answer = Rc::new(Cell::new(Size::ZERO));
+                let root = Frame {
+                    child,
+                    extent,
+                    answer: answer.clone(),
+                }
+                .add(&mut h.rsc);
+                h.set_root(root);
+                (root, leaf, answer)
+            };
+            let mut warm = Harness::new((403, 211));
+            let (root, leaf, answer) = plant(&mut warm, UiRegion::FULL);
+            for width in [191.125, 297.25, 83.75] {
+                let extent =
+                    UiRegion::new(UiSpan::new(Len::px(13.125), Len::px(width)), UiSpan::FULL);
+                warm.rsc[root].extent = extent;
+                warm.frame();
+                let mut cold = Harness::new((403, 211));
+                let (_, other, other_answer) = plant(&mut cold, extent);
+                assert_eq!(answer.get(), other_answer.get());
+                assert_eq!(warm.region(&leaf), cold.region(&other));
+            }
+        }
+    }
+}