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]),
+        }
+    }
+}