Redesign span layout around retained placement

core/src/ui/active.rs

@@ -11,59 +11,15 @@ pub struct ActiveData {
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<PrimitiveHandle>,
     pub children: Vec<WidgetId>,
-    /// The mask this widget was drawn **under** (its parent's), not the
-    /// one it set for itself -- see `own_mask` for that.
+    /// The inherited mask, not `own_mask`.
     pub mask: MaskIdx,
-    /// The mask slot this widget allocated for *itself* with
-    /// `Painter::set_mask`, or `MaskIdx::NONE`. Kept across redraws and
-    /// rewritten in place, the way `move_slot` is: a `Masked` that pushed
-    /// a fresh slot each draw left every already-drawn descendant --
-    /// which `draw_inner`'s unchanged-region fast path does not revisit --
-    /// clipping to the *old* slot's region, so a composer whose bar had
-    /// since been placed at the bottom of the screen was still being
-    /// clipped to a box at the top of it and drew nothing (measured
-    /// 2026-09-06: four mask entries live, none of them the widget's
-    /// current region). Its path out is the `undraw` branch of
-    /// `UiRenderState::remove`, which drops the self-ownership ref taken
-    /// when the slot was allocated.
+    /// The widget's retained mask slot, or `MaskIdx::NONE`.
     pub own_mask: MaskIdx,
     pub layer: LayerId,
-    /// What `Widget::draw` returned the last time this widget was actually
-    /// drawn -- read by a parent placing this widget again without
-    /// redrawing it, replacing `Cache.size`'s old role. See LAYOUT.md
-    /// section 5.
+    /// The last `Widget::draw` result.
     pub size: Size,
-    /// This widget's slot in `UiData::move_offsets`, assigned on its first
-    /// draw and kept for the rest of its life (redraws reuse it in place
-    /// so a retained child's `parent` link never goes stale). See
-    /// LAYOUT.md section 2.
+    /// Retained so descendants' parent links stay valid across redraws.
     pub move_slot: MoveIdx,
-    /// How much of this widget's own `move_slot` delta is already folded
-    /// into `region` above, in window pixels. The two mechanisms that
-    /// write that slot disagree about this and cannot be told apart from
-    /// the slot alone: `UiRenderState::mov` shifts `region` and the delta
-    /// together (the *offered* region genuinely moved), while
-    /// `Painter::reposition` writes only the delta (`region` stays the
-    /// offered box and the delta says where inside it the content was
-    /// placed). So anything that wants the widget's real position --
-    /// `resolved_region`, and through it every hit test -- must subtract
-    /// this from the chain sum. Without it a panned widget's own hit box
-    /// sits at twice the pan while its descendants' are correct, which is
-    /// how it went unnoticed: the composer's field became untappable
-    /// after a finger pan (2026-09-06). Reset to zero whenever the widget
-    /// is really redrawn, since `draw_inner` zeroes the slot then too.
+    /// The part of this widget's move delta already folded into `region`.
     pub move_applied: Vec2,
-    /// The offset the last `Painter::reposition` placed this widget's
-    /// content at *within* `region`, in window pixels. The move slot has
-    /// exactly one owner and one meaning:
-    /// `move_offsets[move_slot] == move_applied + repositioned`. `mov`
-    /// adds to the first, `reposition` overwrites the second (it
-    /// recomputes `from` afresh every call, so repeating it must land on
-    /// the same answer rather than drifting), and both then rewrite the
-    /// slot from the sum -- which is what lets a parent both move a child
-    /// with its own layout and place it inside that moved region in one
-    /// frame. `LazySpan::place`'s Bottom-known branch does exactly that once a
-    /// row's blocks wrap. Reset to zero on a real redraw, with
-    /// `move_applied` and the slot itself.
-    pub repositioned: Vec2,
 }