Measure container children without intermediate placement

core/src/ui/render_state.rs

@@ -193,7 +193,7 @@ impl UiRenderState {
         if let Some(id) = root {
             let region = Self::root_region(id.id(), rsc.widgets());
             let info = self.root_info(region);
-            self.draw_inner(id.id(), region, info, None, rsc);
+            self.draw_inner(id.id(), region, info, None, false, rsc);
         }
     }
 
@@ -213,6 +213,7 @@ impl UiRenderState {
         region: UiRegion,
         info: DrawInfo,
         mut old: Option<ActiveData>,
+        measuring: bool,
         rsc: &mut dyn UiRsc,
     ) -> (Size, LayoutHolds) {
         #[cfg(feature = "layout-diagnostics")]
@@ -239,16 +240,20 @@ impl UiRenderState {
             self.draw_at(id, region, info.offered_placement(), info, old.take(), rsc)
         });
 
-        let declared = declared_lens(rsc.widgets(), id);
         // Where the drawing goes, in the region's own coordinates: what the
         // parent chose, and on any axis it left open, what the answer took of
         // the region placed by the widget's alignment. The region itself does
         // not change, so nothing under it resolves a fraction a second time.
-        let lens = placed_lens(answer.0, declared, info.decided());
-        let own = placed_box(UiRegion::FULL, lens, align);
-        let placement = UiRegion {
-            x: info.placement[0].unwrap_or(own.x),
-            y: info.placement[1].unwrap_or(own.y),
+        let placement = if measuring {
+            info.offered_placement()
+        } else {
+            let declared = declared_lens(rsc.widgets(), id);
+            let lens = placed_lens(answer.0, declared, info.decided());
+            let own = placed_box(UiRegion::FULL, lens, align);
+            UiRegion {
+                x: info.placement[0].unwrap_or(own.x),
+                y: info.placement[1].unwrap_or(own.y),
+            }
         };
         self.place(id, region, placement, info, rsc);
 
@@ -1137,7 +1142,7 @@ impl UiRenderState {
             #[cfg(feature = "layout-diagnostics")]
             diag::bump(Counter::LocalRedraws);
             let old = self.remove(id, false, rsc);
-            self.draw_inner(id, region, info, old, rsc);
+            self.draw_inner(id, region, info, old, false, rsc);
             return true;
         };
         let (given_px, offered_px) = self.asked_px(id);
@@ -1179,9 +1184,9 @@ impl UiRenderState {
             placement: info.offer_placement,
             ..info
         };
-        let answer = self.draw_inner(id, given, offered, old, rsc);
+        let answer = self.draw_inner(id, given, offered, old, false, rsc);
         if info.placement != offered.placement {
-            self.draw_inner(id, given, info, None, rsc);
+            self.draw_inner(id, given, info, None, false, rsc);
         }
         if Some(answer) != was_answer {
             // Its parent chose its box knowing the old answer, so it lays out