Honor fixed child alignment inside parent-selected slots

core/src/ui/painter.rs

@@ -712,16 +712,9 @@ pub(crate) fn placed_box(region: UiRegion, lens: UiVec2, align: RegionAlign) ->
     placed
 }
 
-/// A child's own region and where in it its drawing goes, from the box it is
-/// offered, the lengths its rules declare, and what its parent chose.
-///
-/// A rule gives the region its length outright -- that is what makes a rule
-/// win, and it is why a widget under one never learns of it -- and the region
-/// then sits where its parent placed it, or where its alignment says if its
-/// parent left the axis open. With no rule the region is the whole of what
-/// was offered, since that is the area a fraction under it is a fraction of,
-/// and what the parent chose is where in it the drawing goes. So the two
-/// coordinate spaces are the same one wherever a placement survives.
+/// A declared axis gets a frame of that length, aligned within the parent's
+/// slot (or the offer). Undeclared axes keep the offered frame and chosen
+/// placement, so their reported fractions retain that reference.
 pub(crate) fn ask_box(
     mut region: UiRegion,
     declared: [Option<LayoutLen>; 2],
@@ -736,10 +729,8 @@ pub(crate) fn ask_box(
         };
         let span = region.axis_mut(axis);
         let len = Len::from_parts(len.rel, len.px);
-        span.start = match chosen {
-            Some(chosen) => chosen.start,
-            None => span.start + (span.len() - len).scale(align.axis(axis).rel()),
-        };
+        let slot = chosen.unwrap_or(*span);
+        span.start = slot.start + (slot.len() - len).scale(align.axis(axis).rel());
         span.end = span.start + len;
     }
     (region, placed)

tests/cases/layout.rs

@@ -716,3 +716,24 @@ fn a_stack_sized_by_a_child_does_not_take_that_childs_fraction_twice() {
     assert_corners!(h, half, (0, 0), (200, 200));
     assert_corners!(h, behind, (0, 0), (200, 200));
 }
+
+#[test]
+fn a_fixed_child_is_centered_in_its_wrappers_share() {
+    let mut h = Harness::new((600, 300));
+    let leaf = rect(Color::RED).sized((100, 100)).center().add(&mut h.rsc);
+    let wrapper = leaf
+        .wrapper()
+        .width(leftover(2))
+        .height(rel(1.0))
+        .add(&mut h.rsc);
+    let other = rect(Color::BLUE).width(200).add(&mut h.rsc);
+    h.set_root((other, wrapper).span(Dir::RIGHT));
+
+    assert_corners!(h, wrapper, (200, 0), (600, 300));
+    assert_corners!(h, leaf, (350, 100), (450, 200));
+
+    h.resize((900, 400));
+    h.frame();
+    assert_corners!(h, wrapper, (200, 0), (900, 400));
+    assert_corners!(h, leaf, (500, 150), (600, 250));
+}