Changeset 260828 in webkit

Timestamp:

Apr 28, 2020 9:05:36 AM (4 years ago)

Author:

Antti Koivisto

Message:

msn.com: Header flickers when scrolling articles
​https://bugs.webkit.org/show_bug.cgi?id=211126
<rdar://problem/56439177>

Reviewed by Simon Fraser.

Source/WebCore:

Test: compositing/fixed-with-clip-stability.html

In case of fixed positioned elements the decision to create backing depends on clip rect.
However RenderLayer::localClipRect() tests for backing in call to clippingRootForPainting(). This creates
instability since clipping depends on backing decision, and backing decision depends on clipping.

• rendering/RenderLayer.cpp:

(WebCore::RenderLayer::localClipRect const):

Specifically the result of clipExceedsBounds test here is affected by computed offsetFromRoot:
"clipRect.contains(cssClipRect)" test fails for zero sized clips with different offsets.

Compute clipExceedsBounds by looking at the clip sizes only and ignoring the position (which should match).

LayoutTests:

• compositing/fixed-with-clip-stability-expected.txt: Added.
• compositing/fixed-with-clip-stability.html: Added.

Location:

trunk

Files:

• LayoutTests/ChangeLog (modified) (1 diff)
• LayoutTests/compositing/fixed-with-clip-stability-expected.txt (added)
• LayoutTests/compositing/fixed-with-clip-stability.html (added)
• Source/WebCore/ChangeLog (modified) (1 diff)
• Source/WebCore/rendering/RenderLayer.cpp (modified) (1 diff)

trunk/LayoutTests/ChangeLog

@@ +1 @@
+2020-04-28  Antti Koivisto  <antti@apple.com>
+
+        msn.com: Header flickers when scrolling articles
+        https://bugs.webkit.org/show_bug.cgi?id=211126
+        <rdar://problem/56439177>
+
+        Reviewed by Simon Fraser.
+
+        * compositing/fixed-with-clip-stability-expected.txt: Added.
+        * compositing/fixed-with-clip-stability.html: Added.
+
 2020-04-28  Jason Lawrence  <lawrence.j@apple.com>
 

trunk/Source/WebCore/ChangeLog

@@ +1 @@
+2020-04-28  Antti Koivisto  <antti@apple.com>
+
+        msn.com: Header flickers when scrolling articles
+        https://bugs.webkit.org/show_bug.cgi?id=211126
+        <rdar://problem/56439177>
+
+        Reviewed by Simon Fraser.
+
+        Test: compositing/fixed-with-clip-stability.html
+
+        In case of fixed positioned elements the decision to create backing depends on clip rect.
+        However RenderLayer::localClipRect() tests for backing in call to clippingRootForPainting(). This creates
+        instability since clipping depends on backing decision, and backing decision depends on clipping.
+
+        * rendering/RenderLayer.cpp:
+        (WebCore::RenderLayer::localClipRect const):
+
+        Specifically the result of clipExceedsBounds test here is affected by computed offsetFromRoot:
+        "clipRect.contains(cssClipRect)" test fails for zero sized clips with different offsets.
+
+        Compute clipExceedsBounds by looking at the clip sizes only and ignoring the position (which should match).
+
 2020-04-28  Zalan Bujtas  <zalan@apple.com>
 

trunk/Source/WebCore/rendering/RenderLayer.cpp

@@ -5979 +5979 @@
     if (renderer().hasClip()) {
         // CSS clip may be larger than our border box.
-        LayoutRect cssClipRect = downcast<RenderBox>(renderer()).clipRect(toLayoutPoint(offsetFromRoot), nullptr);
-        clipExceedsBounds = !clipRect.contains(cssClipRect);
+        LayoutRect cssClipRect = downcast<RenderBox>(renderer()).clipRect({ }, nullptr);
+        clipExceedsBounds = !cssClipRect.isEmpty() && (clipRect.width() < cssClipRect.width() || clipRect.height() < cssClipRect.height());
     }