Avoid touching z-index in StyleAdjuster by using an isStackingContext flag instead

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

 # `Source/core/paint`
 
 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.
 
 According to the documentation, we can have the following types of elements that are
 treated in different ways during painting:
 
 *   Stacked objects: objects that are z-ordered in stacking contexts, including:
 
-    *   Stacking contexts: elements with non-auto z-indices.
+    *   Stacking contexts: elements with non-auto z-indices or other properties
+        that affect stacking e.g. transform, opacity, blend-mode.
 
     *   Elements that are not real stacking contexts but are treated as stacking
         contexts but don't manage other stacked elements. Their z-ordering are
         managed by real stacking contexts. They are positioned elements with
         `z-index: auto` (E.2.8 in the documentation).
 
         They must be managed by the enclosing stacking context as stacked elements
         because `z-index:auto` and `z-index:0` are considered equal for stacking
         context sorting and they may interleave by DOM order.
 
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 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.