Implement will-change-based creation of layers, stacking contexts, and containing blocks

Source/core/css/resolver/StyleAdjuster.cpp

Index: Source/core/css/resolver/StyleAdjuster.cpp
diff --git a/Source/core/css/resolver/StyleAdjuster.cpp b/Source/core/css/resolver/StyleAdjuster.cpp
index 7e8b3ee4309acba66f1e09e6ec046b116d58e71b..d426be1465e3c5f3da4b3018b2e93a78889e052e 100644
--- a/Source/core/css/resolver/StyleAdjuster.cpp
+++ b/Source/core/css/resolver/StyleAdjuster.cpp
@@ -44,6 +44,7 @@
 #include "core/rendering/style/RenderStyle.h"
 #include "core/rendering/style/RenderStyleConstants.h"
 #include "platform/Length.h"
+#include "platform/transforms/TransformOperations.h"
 #include "wtf/Assertions.h"
 
 namespace WebCore {
@@ -158,6 +159,34 @@ static bool parentStyleForcesZIndexToCreateStackingContext(const RenderStyle* pa
     return isDisplayFlexibleBox(parentStyle->display()) || isDisplayGridBox(parentStyle->display());
 }
 
+static bool hasWillChangeThatCreatesStackingContext(const RenderStyle* style, Element* e)
+{
+    for (size_t i = 0; i < style->willChangeProperties().size(); ++i) {
+        switch (style->willChangeProperties()[i]) {
+        case CSSPropertyOpacity:
+        case CSSPropertyWebkitTransform:
+        case CSSPropertyWebkitTransformStyle:
+        case CSSPropertyWebkitPerspective:
+        case CSSPropertyWebkitMask:
+        case CSSPropertyWebkitMaskBoxImage:
+        case CSSPropertyWebkitClipPath:
+        case CSSPropertyWebkitBoxReflect:
+        case CSSPropertyWebkitFilter:
+        case CSSPropertyZIndex:
+        case CSSPropertyPosition:

[jamesr, 2014/02/26 23:56:22]

this is supposed to be a set of all CSS properties that can in any value create
a stacking context? it seems very fragile to keep this set here in just a switch
- are you sure this has all cases?

[ajuma, 2014/02/27 00:05:47]

Yes, this is supposed to be all CSS properties that have some value that creates
a stacking context. The list here corresponds to the conditions under which
adjustRenderStyle calls setZIndex(0) (see lines 295-309 below), so should be the
complete list.

[esprehn, 2014/02/27 20:58:31]

This just begs for a unit test, but I suppose it'll be hard to do.

+            return true;
+        case CSSPropertyMixBlendMode:
+        case CSSPropertyIsolation:
+            if (RuntimeEnabledFeatures::cssCompositingEnabled())
+                return true;
+            break;
+        default:
+            break;
+        }
+    }
+    return false;
+}
+
 void StyleAdjuster::adjustRenderStyle(RenderStyle* style, RenderStyle* parentStyle, Element *e)
 {
     ASSERT(parentStyle);
@@ -275,9 +304,17 @@ void StyleAdjuster::adjustRenderStyle(RenderStyle* style, RenderStyle* parentSty
         || style->position() == StickyPosition
         || (style->position() == FixedPosition && e && e->document().settings() && e->document().settings()->fixedPositionCreatesStackingContext())
         || isInTopLayer(e, style)
+        || hasWillChangeThatCreatesStackingContext(style, e)
         ))
         style->setZIndex(0);
 
+    // will-change:transform should result in the same rendering behavior as having a transform,
+    // including the creation of a containing block for fixed position descendants.

[esprehn, 2014/02/27 22:05:27]

This is observable from script. If you do getComputedStyle(e).transform you'll
see the identity transform you've added. That's fine, but we need to make the
spec text say something like  "The transform of the element computes to the
identity if you used will-animate: transform"

+    if (!style->hasTransform() && style->willChangeProperties().contains(CSSPropertyWebkitTransform)) {
+        bool makeIdentity = true;
+        style->setTransform(TransformOperations(makeIdentity));
+    }
+
     // Textarea considers overflow visible as auto.
     if (e && e->hasTagName(textareaTag)) {
         style->setOverflowX(style->overflowX() == OVISIBLE ? OAUTO : style->overflowX());