cross‑frame pre‑paint traversal.

Runs in the InPrePaint lifecycle phase. Walks the entire layout tree starting at the root LocalFrameView, updating paint invalidation state and computing the context needed by the tree‑building step (e.g., event‑handler flags, fragmentation context, containers for out‑of‑flow). (Chromium Git Repositories)

Key types & fields

* void WalkTree(LocalFrameView& root_frame) — entry to pre‑paint traversal. (Chromium Git Repositories)

* struct PrePaintTreeWalkContext — per‑walk state (e.g., paint_invalidator_context, inside_blocking_touch_event_handler, fragmentation containers). These flags determine whether descendants need updates. (Chromium Git Repositories)

Hands off to: PaintPropertyTreeBuilder and then painting.