Redesign span layout around retained placement

src/layout_tests.rs

@@ -23,6 +23,51 @@ impl UiRsc for TestRsc {
     }
 }
 
+struct FixedRect(f32);
+
+impl Widget for FixedRect {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let size = Size::from_axis(Axis::Y, Len::abs(self.0), Len::REST);
+        painter.primitive_within(
+            RectPrimitive::color(UiColor::WHITE),
+            size.to_uivec2(painter.density())
+                .align(RegionAlign::TOP_LEFT),
+        );
+        size
+    }
+}
+
+#[test]
+fn a_hinted_rest_draws_once_and_only_moves_the_fixed_child_after_it() {
+    let mut rsc = TestRsc {
+        ui: UiData::default(),
+    };
+    let first = rsc.ui.widgets.add_strong(FixedRect(40.0));
+    let fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
+    let fill = rsc.ui.widgets.add_strong(Sized {
+        inner: fill.any(),
+        x: None,
+        y: Some(Len::REST),
+    });
+    let last = rsc.ui.widgets.add_strong(FixedRect(40.0));
+    let last_w = last.weak();
+    let mut span = Span::empty(Dir::DOWN);
+    span.push(first.any());
+    span.push(fill.any());
+    span.push(last.any());
+    let root = rsc.ui.widgets.add_strong(span).any();
+
+    let mut render = UiRenderState::new();
+    render.resize((200.0, 300.0));
+    render.update(&root, &mut rsc);
+    let (draws, _rewrites, moves, _shapes) = render.take_counters();
+
+    assert_eq!(draws, 5);
+    assert_eq!(moves, 1);
+    let last = render.window_region(&last_w, &rsc).unwrap();
+    assert!((last.top_left.y - 260.0).abs() < 0.01, "{last:?}");
+}
+
 /// A `ScrollArea` over a `Span` of `n` fixed-height rects -- N primitives large
 /// enough that an O(N) regression in the move path would show up as a
 /// non-trivial counter rather than being lost in noise (LAYOUT.md section
@@ -227,7 +272,7 @@ fn a_mask_stays_put_while_its_scrolled_content_moves() {
     let masked_slot_after = render.active.get(&masked_id).unwrap().move_slot;
     let mask_delta_after = rsc.ui.move_offsets[masked_slot_after.idx()].delta;
 
-    // `Masked` itself is never the target of a `mov`/`reposition` here --
+    // `Masked` itself is never the target of a `mov`/`place` here --
     // only its scrolled child is -- so the slot its own mask references
     // (`Painter::set_mask` bakes in `self.move_slot`, i.e. this one) must
     // still read zero after the scroll. The visible counterpart of this
@@ -307,7 +352,7 @@ fn composing_text_after_a_keyboard_resize_lands_in_the_bars_own_region() {
     // The keyboard opens: a real `surface_changed`/`resize` to a shorter
     // window, then a further keystroke -- the redraw that must land in the
     // bar's new (also short) region, not whatever region a provisional
-    // measurement pass used along the way.
+    // provisional placement used along the way.
     render.resize((1080.0, 1478.0));
     render.update(&root, &mut rsc);
     field.edit(&mut rsc).insert("b");
@@ -583,9 +628,6 @@ fn a_size_independent_widget_moved_by_its_parent_has_the_hit_box_it_is_drawn_at(
     let mut render = UiRenderState::new();
     render.resize((800.0, 600.0));
     render.update(&root, &mut rsc);
-    // `Span` draws each child once at the full region to measure it and
-    // then places it, so this widget has already been through the branch
-    // once by the end of the very first frame.
     let first = render.window_region(&below_w, &rsc).unwrap();
     assert!(
         (first.top_left.y - 100.0).abs() < 0.01,
@@ -604,7 +646,7 @@ fn a_size_independent_widget_moved_by_its_parent_has_the_hit_box_it_is_drawn_at(
 }
 
 /// A parent that both `mov`s a child (its own layout moved the box it
-/// offers) and `reposition`s it inside that box in the same frame -- what
+/// offers) and places it inside that box in the same frame -- what
 /// `LazySpan::place`'s Bottom-known branch does once a row's cached height
 /// stops matching what the row reports, which is reachable as soon as a
 /// transcript row's blocks wrap (docs/IRIS_TODO.md's "Found by P1a").
@@ -634,21 +676,12 @@ impl Widget for MoveThenPlace {
                 UiScalar::abs(self.place_top + 40.0),
             ),
         );
-        painter.reposition(&self.inner, place);
+        painter.place(&self.inner, place);
         Size::default()
     }
 }
 
-/// `mov` accumulates a delta onto a widget's move slot and `reposition`
-/// overwrites it, and both can legitimately land on one widget in one
-/// frame (see `MoveThenPlace`). `reposition` used to write its own delta
-/// alone, which dropped the move and put the child back at the position
-/// the offered box had *before* it moved; a `debug_assert!` that
-/// `move_applied` was zero hid that behind a panic instead of fixing it.
-/// The slot has one owner and one meaning now --
-/// `move_applied + repositioned` -- so the child stays where it was
-/// placed however its offered box moves. Fails at the offer's position
-/// (200) rather than the placement's (100) without that.
+/// Placement must preserve a move already applied in the same frame.
 #[test]
 fn a_widget_moved_by_its_parent_and_then_placed_inside_it_lands_at_the_placement() {
     let mut rsc = TestRsc {
@@ -1025,6 +1058,7 @@ fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
         let padded = rsc.ui.widgets.add_strong(Pad {
             padding: Padding::uniform(PAD),
             inner: sized.any(),
+            exact_region: false,
         });
         let fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::BLUE)).any();
         let card = rsc.ui.widgets.add_strong(Stack {