Changeset 152335 in webkit

Timestamp:

Jul 2, 2013 8:19:06 PM (11 years ago)

Author:

Simon Fraser

Message:

Don't set z-index: 0 on lots of elements with -webkit-overflow-scrolling: touch
​https://bugs.webkit.org/show_bug.cgi?id=118337

Reviewed by Benjamin Poulain.

-webkit-overflow-scrolling: touch is an inherited property that, on iOS, controls
the behavior of overflow scrolling content, and causes overflow scrolling elements
to become CSS stacking contexts.

However, the code was too aggressive in setting style->setZIndex(0), doing so
for any element with overflow != hidden. Since the default for overflow is visible,
that meant almost every element.

Previously, this didn't really matter. However, since r125693, any renderer with non-auto
z-index gets a RenderLayer, and that RenderLayer will become a stacking context. The result
was too many RenderLayers and incorrect stacking context behavior.

Fix by ensuring that -webkit-overflow-scrolling: touch only affects elements which
are actually scrollable.

Also move the code that does this to below the code that adjust overflow style
for different elements.

• css/StyleResolver.cpp:

(WebCore::isScrollableOverflow):
(WebCore::StyleResolver::adjustRenderStyle):

Location:

trunk/Source/WebCore

Files:

• ChangeLog (modified) (1 diff)
• css/StyleResolver.cpp (modified) (3 diffs)

trunk/Source/WebCore/ChangeLog

@@ +1 @@
+2013-07-02  Simon Fraser  <simon.fraser@apple.com>
+
+        Don't set z-index: 0 on lots of elements with -webkit-overflow-scrolling: touch
+        https://bugs.webkit.org/show_bug.cgi?id=118337
+
+        Reviewed by Benjamin Poulain.
+
+        -webkit-overflow-scrolling: touch is an inherited property that, on iOS, controls
+        the behavior of overflow scrolling content, and causes overflow scrolling elements
+        to become CSS stacking contexts.
+         
+        However, the code was too aggressive in setting style->setZIndex(0), doing so
+        for any element with overflow != hidden. Since the default for overflow is visible,
+        that meant almost every element.
+        
+        Previously, this didn't really matter. However, since r125693, any renderer with non-auto
+        z-index gets a RenderLayer, and that RenderLayer will become a stacking context. The result
+        was too many RenderLayers and incorrect stacking context behavior.
+        
+        Fix by ensuring that -webkit-overflow-scrolling: touch  only affects elements which
+        are actually scrollable.
+        
+        Also move the code that does this to below the code that adjust overflow style
+        for different elements.
+
+        * css/StyleResolver.cpp:
+        (WebCore::isScrollableOverflow):
+        (WebCore::StyleResolver::adjustRenderStyle):
+
 2013-07-02  Commit Queue  <commit-queue@webkit.org>
 

trunk/Source/WebCore/css/StyleResolver.cpp

@@ -1353 +1353 @@
 }
 
+#if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
+static bool isScrollableOverflow(EOverflow overflow)
+{
+    return overflow == OSCROLL || overflow == OAUTO || overflow == OOVERLAY;
+}
+#endif
+
 void StyleResolver::adjustRenderStyle(RenderStyle* style, RenderStyle* parentStyle, Element *e)
 {
@@ -1471 +1478 @@
         || style->position() == StickyPosition
         || (style->position() == FixedPosition && e && e->document()->page() && e->document()->page()->settings()->fixedPositionCreatesStackingContext())
-#if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
-        // Touch overflow scrolling creates a stacking context.
-        || ((style->overflowX() != OHIDDEN || style->overflowY() != OHIDDEN) && style->useTouchOverflowScrolling())
-#endif
 #if ENABLE(DIALOG_ELEMENT)
         || (e && e->isInTopLayer())
@@ -1526 +1529 @@
         style->setOverflowY(OVISIBLE);
     }
+
+#if ENABLE(ACCELERATED_OVERFLOW_SCROLLING)
+    // Touch overflow scrolling creates a stacking context.
+    if (style->useTouchOverflowScrolling() && (isScrollableOverflow(style->overflowX()) || isScrollableOverflow(style->overflowY())))
+        style->setZIndex(0);
+#endif
 
     // Cull out any useless layers and also repeat patterns into additional layers.