Constrain offered boxes with independent widget size bounds

core/src/ui/render_state.rs

@@ -254,7 +254,6 @@ impl UiRenderState {
             );
         }
         let align = rsc.widgets().alignment(id);
-        let declared = info.declared;
         // Nothing this widget measured can be dirty while it draws: layout is
         // one bottom-up walk, so anything deeper has settled or deferred to
         // its own parent, and a deferred one leaves that parent marked.
@@ -268,7 +267,7 @@ impl UiRenderState {
             .then(|| self.retained_answer(id, region, info))
             .flatten()
             .and_then(|answer| {
-                let placed = info.placed.placement(region, answer.size, declared, align);
+                let placed = info.placed.placement(region, answer.size, align);
                 self.try_reuse(id, region, placed, info, rsc)
                     .then_some(answer)
             });
@@ -280,7 +279,7 @@ impl UiRenderState {
             // Where the drawing goes: the part its parent gave it, with the
             // answer placed inside that part on any axis the parent left
             // open.
-            let placed = info.placed.placement(region, answer.size, declared, align);
+            let placed = info.placed.placement(region, answer.size, align);
             if placed != region {
                 self.relocate(id, placed, info, rsc);
             }
@@ -379,10 +378,7 @@ impl UiRenderState {
             size_deps,
             request_deps,
             scratch,
-            // What the ask holds for is part of what the drawing holds for:
-            // a box the widget's own rule took past the offer was decided in
-            // this window, and at the root nobody else keeps that range.
-            own: info.ask_holds,
+            own: LayoutHolds::ANY,
             under,
             answer_under: LayoutHolds::ANY,
             depth: info.depth,
@@ -437,9 +433,9 @@ impl UiRenderState {
         // rel base is the answer wherever the ask declared a length: it was
         // resolved into the rel base when the widget was asked, and resolving
         // it again here would take the fraction of a fraction.
-        let rules = rsc.widgets().size_rules(id).clone();
+        let rules = rsc.widgets().size_rules(id);
         let ruled = |axis: Axis, reported: LayoutLen| {
-            if matches!(rules[axis], crate::SizeRule::Request(_)) {
+            if rules[axis].deferred().is_some() {
                 return info.rel_base[axis].into();
             }
             match rules[axis].exact() {
@@ -470,18 +466,30 @@ impl UiRenderState {
         // it is a length only to whoever divides one, and the box that
         // divider gives is a box this widget is asked in, where the bound is
         // applied to it.
-        let mut bounded = LayoutHolds::ANY;
+        // Widgets may widen their own read ranges, but not the ask's constraints.
+        let mut own_holds = own.and(info.ask_holds);
         for axis in Axis::BOTH {
             let answer = size[axis];
             if answer.leftover != Weight::ZERO {
                 continue;
             }
             let (held, kept) = info.bounds[axis].outside(answer.without_leftover(), window[axis]);
-            bounded[axis].window = kept;
+            own_holds[axis].window = own_holds[axis].window.and(kept);
             if let Some(held) = held {
                 size[axis] = held.into();
             }
         }
+        // A rule that is a fraction of the rel base is answered with the
+        // rel base's own length, so the answer is that rel base's and not just
+        // that many pixels of this window -- the same pin a widget that read
+        // its rel base took for its drawing. A bound counts: which side of it
+        // the box fell was decided against this rel base, and the same box of
+        // a different one can fall on the other.
+        for axis in Axis::BOTH {
+            if rules[axis].has_fraction() {
+                own_holds[axis].rel_base = Some(info.rel_base[axis]);
+            }
+        }
         // A widget that clipped its contents to its box drew nothing outside
         // it, so reporting more than the box asks to be placed at a length it
         // does not occupy -- and its parent would place the part it cut off.
@@ -509,18 +517,6 @@ impl UiRenderState {
         if let Some(idx) = retired_move {
             self.moves.remove(idx);
         }
-        // A rule that is a fraction of the rel base is answered with the
-        // rel base's own length, so the answer is that rel base's and not just
-        // that many pixels of this window -- the same pin a widget that read
-        // its rel base took for its drawing. A bound counts: which side of it
-        // the box fell was decided against this rel base, and the same box of
-        // a different one can fall on the other.
-        let mut own_holds = own.and(bounded);
-        for axis in Axis::BOTH {
-            if rules[axis].has_fraction() {
-                own_holds[axis].rel_base = Some(info.rel_base[axis]);
-            }
-        }
         let answer_holds = own_holds.and(answer_under);
         let holds = under
             .iter()
@@ -798,7 +794,6 @@ impl UiRenderState {
         let placed = place.placement(
             region,
             active.measured().unwrap_or(active.size),
-            active.declared,
             active.own_align,
         );
         let info = DrawInfo {
@@ -823,8 +818,8 @@ impl UiRenderState {
 
     /// The rel base and the box a widget already drawn is given at `place` of
     /// the box its parent is being taken as. What narrowed its rel base and what
-    /// it declared are its own record's, so both are resolved against that
-    /// parent's rel base again exactly as the first ask resolved them.
+    /// it declared are its own record's. Declared lengths stay as the ask
+    /// resolved them; the destination only decides where they sit.
     fn ask_again(active: &ActiveData, at: &Placing, place: PlaceDesc) -> (UiVec2, UiRegion) {
         place.rel_base_and_region(at.region, at.rel_base, active.declared, active.own_align)
     }
@@ -1195,10 +1190,11 @@ impl UiRenderState {
             active.asked,
         );
         let active = &self.active[&id];
-        let declared_changed = ask.declared != active.declared;
+        // Even an inactive bound changes what the parent must track about its offer.
+        let constraints_changed = ask.declared != active.declared || ask.bounds != active.bounds;
         let alignment_changed = rsc.widgets().alignment(id) != active.own_align;
         if let Some(parent) = active.parent
-            && (declared_changed
+            && (constraints_changed
                 || alignment_changed
                 || active.re_asked
                 || !active.drawn