InPrePaint document state and PrePaintTreeWalk class

third_party/WebKit/Source/core/paint/README.md

 # `Source/core/paint`
 
-This directory contains implementation of painters of layout objects.
+This directory contains implementation of painters of layout objects. It covers
+the following document lifecycle states:
+
+*   PaintInvalidation (`InPaintInvalidation` and `PaintInvalidationClean`)
+*   PrePaint (`InPrePaint` and `PrePaintClean`)
+*   Paint (`InPaint` and `PaintClean`)
 
 ## Glossaries
 
 ### Stacked elements and stacking contexts
 
 This chapter is basically a clarification of [CSS 2.1 appendix E. Elaborate description
 of Stacking Contexts](http://www.w3.org/TR/CSS21/zindex.html).
 
 Note: we use 'element' instead of 'object' in this chapter to keep consistency with
 the spec. We use 'object' in other places in this document.
@@ skipping 167 matching lines @@
 *   The normal paint invalidation is based on whole LayoutObject's, not aware of the first line.
 
 We have a special path for first line style change: the style system informs the layout system
 when the computed first-line style changes through `LayoutObject::firstLineStyleDidChange()`.
 When this happens, we invalidate all `InlineBox`es in the first line.
 
 ### Slimming paint v2
 
 TODO(wangxianzhu): add details
 
+## [`PrePaintTreeWalk`](PrePaintTreeWalk.h) (Slimming paint v2 only)
+
+During `InPrePaint` document lifecycle state, this class is called to walk the whole
+layout tree, beginning from the root FrameView, across frame boundaries. We do the
+following during the tree walk:
+
+*   Building paint property tree: creates paint property tree nodes for special
+    things in the layout tree, including but not limit to: overflow clip, transform,
+    fixed-pos, animation, mask, filter, etc.
+
+*   Paint invalidation: Not implemented yet. TODO(wangxianzhu): add details after
+    it's implemented.
+
 ## Paint result caching
 
 `PaintController` holds the previous painting result as a cache of display items.
 If some painter would generate results same as those of the previous painting,
 we'll skip the painting and reuse the display items from cache.
 
 ### Display item caching
 
 We'll create a `CachedDisplayItem` when a painter would create a `DrawingDisplayItem` exactly
 the same as the display item created in the previous painting. After painting, `PaintController`
@@ skipping 34 matching lines @@
 
 When layer structure changes, and we are not invalidate paint of the changed subtree,
 we need to manually update the `needsPaintPhaseXXX` flags. For example, if an object changes
 style and creates a self-painting-layer, we copy the flags from its containing self-painting
 layer to this layer, assuming that this layer needs all paint phases that its container
 self-painting layer needs.
 
 We could update the `needsPaintPhaseXXX` flags in a separate tree walk, but that would regress
 performance of the first paint. For slimming paint v2, we can update the flags during the
 pre-painting tree walk to simplify the logics.