Rename isTreatedAsOrStackingContext to isStacked

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

Index: third_party/WebKit/Source/core/paint/README.md
diff --git a/third_party/WebKit/Source/core/paint/README.md b/third_party/WebKit/Source/core/paint/README.md
index 049c92e457f758de557398e3fa9ed3476700f577..fa036e54c41ddae80063d1de6d87a5b9403da0db 100644
--- a/third_party/WebKit/Source/core/paint/README.md
+++ b/third_party/WebKit/Source/core/paint/README.md
@@ -2,6 +2,46 @@
 
 This directory contains implementation of painters of layout objects.
 
+## Stacked contents 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).
+  
+According to the documentation, we can have the following types of elements that are
+treated in different ways during painting:
+
+*   Stacked elements: elements that are z-ordered in stacking contexts.
+
+    *   Stacking contexts: elements with non-auto z-indices.
+
+    *   Elements that are not real stacking contexts but are treated as stacking
+        contexts and their z-ordering are managed by real stacking contexts.
+        They are positioned elements with `z-index: auto` (E.2.8 in the documentation).

[chrishtr, 2016/03/16 16:26:16]

Add a paragraph below that explains what "treated as stacking contexts" means.
A reasonable summary may be:

Execute all of the steps of the painting algorithm explained in
https://www.w3.org/TR/CSS21/zindex.html for this element, except ignore any
descendants which are positioned or have non-auto z-index.

[Xianzhu, 2016/03/16 18:10:24]

Done.

+        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.
+
+    They are painted as self-painting PaintLayers.
+
+*   Non-stacked pseudo stacking contexts: elements that are not stacked, but paint
+    their descendants (excluding any stacked contents) as if they created stacking
+    contexts. This includes
+
+    *   inline blocks, inline tables, inline-level replaced elements
+        (E.2.7.2.1.4 in the documentation)
+    *   non-positioned floating elements (E.2.5 in the documentation)
+    *   [flex items](http://www.w3.org/TR/css-flexbox-1/#painting)
+    *   [grid items](http://www.w3.org/TR/css-grid-1/#z-order)
+    *   custom scrollbar parts
+
+    They are painted as pseudo stacking contexts by `ObjectPainter::paintAsPseudoStackingContext()`.

[pdr., 2016/03/16 02:22:53]

Nit: update to be paintAllPhasesAtomically

[Xianzhu, 2016/03/16 18:10:24]

Done.

+
+*   Other normal elements.
+
+"Stacked element" is not defined as a formal term in the documentation, but we found
+it convenient to use this term to refer to any elements participating z-index ordering
+in stacking contexts.
+
 ## Painters
 
 ## Paint invalidation