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<dl class="meta">
<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/265327">265327</a></dd>
<dt>Author</dt> <dd>simon.fraser@apple.com</dd>
<dt>Date</dt> <dd>2020-08-06 07:55:55 -0700 (Thu, 06 Aug 2020)</dd>
</dl>

<h3>Log Message</h3>
<pre>Scrolling tree nodes sometimes don't match layer z-order
https://bugs.webkit.org/show_bug.cgi?id=215210

Reviewed by Antti Koivisto.

Source/WebCore:

When adding nodes to the scrolling state tree during compositing layer traversal,
we would sometimes add then in the wrong order. For example, if the composited layer
tree was:

+ Root
  + Stacking context
    Fixed layer 1
  Fixed layer 2

we would build a scrolling tree like:

+ Root scrolling node
  Node for layer 2
  Node for layer 1

This happened because RenderLayerCompositor::updateBackingAndHierarchy() failed to propagate up
scrollingTreeState.nextChildIndex.

This is generally benign, but inserting node 1 followed by node 2 would be seen as a scrolling tree
mutation, causing us to re-commit the scrolling tree when it hadn't really changed, triggering
extra CPU usage.

Part of <rdar://problem/62737868>: higher CPU usage on instagram.com.

Test: scrollingcoordinator/scrolling-tree/sibling-node-order.html

* rendering/RenderLayerCompositor.cpp:
(WebCore::RenderLayerCompositor::updateBackingAndHierarchy):

LayoutTests:

* platform/ios-wk2/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt: Added.
* scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt: Added.
* scrollingcoordinator/scrolling-tree/sibling-node-order.html: Added.</pre>

<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkLayoutTestsChangeLog">trunk/LayoutTests/ChangeLog</a></li>
<li><a href="#trunkSourceWebCoreChangeLog">trunk/Source/WebCore/ChangeLog</a></li>
<li><a href="#trunkSourceWebCorerenderingRenderLayerCompositorcpp">trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp</a></li>
</ul>

<h3>Added Paths</h3>
<ul>
<li><a href="#trunkLayoutTestsplatformioswk2scrollingcoordinatorscrollingtreesiblingnodeorderexpectedtxt">trunk/LayoutTests/platform/ios-wk2/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt</a></li>
<li><a href="#trunkLayoutTestsscrollingcoordinatorscrollingtreesiblingnodeorderexpectedtxt">trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt</a></li>
<li><a href="#trunkLayoutTestsscrollingcoordinatorscrollingtreesiblingnodeorderhtml">trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order.html</a></li>
</ul>

</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkLayoutTestsChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/LayoutTests/ChangeLog (265326 => 265327)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/ChangeLog      2020-08-06 14:48:07 UTC (rev 265326)
+++ trunk/LayoutTests/ChangeLog 2020-08-06 14:55:55 UTC (rev 265327)
</span><span class="lines">@@ -1,3 +1,14 @@
</span><ins>+2020-08-06  Simon Fraser  <simon.fraser@apple.com>
+
+        Scrolling tree nodes sometimes don't match layer z-order
+        https://bugs.webkit.org/show_bug.cgi?id=215210
+
+        Reviewed by Antti Koivisto.
+
+        * platform/ios-wk2/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt: Added.
+        * scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt: Added.
+        * scrollingcoordinator/scrolling-tree/sibling-node-order.html: Added.
+
</ins><span class="cx"> 2020-08-06  Wenson Hsieh  <wenson_hsieh@apple.com>
</span><span class="cx"> 
</span><span class="cx">         Remove UIScriptController.removeAllDynamicDictionaries()
</span></span></pre></div>
<a id="trunkLayoutTestsplatformioswk2scrollingcoordinatorscrollingtreesiblingnodeorderexpectedtxt"></a>
<div class="addfile"><h4>Added: trunk/LayoutTests/platform/ios-wk2/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt (0 => 265327)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/platform/ios-wk2/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt                           (rev 0)
+++ trunk/LayoutTests/platform/ios-wk2/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt      2020-08-06 14:55:55 UTC (rev 265327)
</span><span class="lines">@@ -0,0 +1,28 @@
</span><ins>+Top
+Bottom
+
+(Frame scrolling node
+  (scrollable area size 800 600)
+  (contents size 800 600)
+  (scrollable area parameters 
+    (horizontal scroll elasticity 1)
+    (vertical scroll elasticity 1)
+    (horizontal scrollbar mode 0)
+    (vertical scrollbar mode 0))
+  (layout viewport at (0,0) size 800x600)
+  (min layout viewport origin (0,0))
+  (max layout viewport origin (0,0))
+  (behavior for fixed 0)
+  (children 2
+    (Fixed node
+      (anchor edges: AnchorEdgeLeft AnchorEdgeTop)
+      (viewport rect at last layout at (0,0) size 800x600)
+    )
+    (Fixed node
+      (anchor edges: AnchorEdgeLeft AnchorEdgeBottom)
+      (viewport rect at last layout at (0,0) size 800x600)
+      (layer position at last layout (0,500))
+    )
+  )
+)
+
</ins></span></pre></div>
<a id="trunkLayoutTestsscrollingcoordinatorscrollingtreesiblingnodeorderexpectedtxt"></a>
<div class="addfile"><h4>Added: trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt (0 => 265327)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt                            (rev 0)
+++ trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order-expected.txt       2020-08-06 14:55:55 UTC (rev 265327)
</span><span class="lines">@@ -0,0 +1,28 @@
</span><ins>+Top
+Bottom
+
+(Frame scrolling node
+  (scrollable area size 800 600)
+  (contents size 800 600)
+  (scrollable area parameters 
+    (horizontal scroll elasticity 2)
+    (vertical scroll elasticity 2)
+    (horizontal scrollbar mode 0)
+    (vertical scrollbar mode 0))
+  (layout viewport at (0,0) size 800x600)
+  (min layout viewport origin (0,0))
+  (max layout viewport origin (0,0))
+  (behavior for fixed 0)
+  (children 2
+    (Fixed node
+      (anchor edges: AnchorEdgeLeft AnchorEdgeTop)
+      (viewport rect at last layout at (0,0) size 800x600)
+    )
+    (Fixed node
+      (anchor edges: AnchorEdgeLeft AnchorEdgeBottom)
+      (viewport rect at last layout at (0,0) size 800x600)
+      (layer position at last layout (0,500))
+    )
+  )
+)
+
</ins></span></pre></div>
<a id="trunkLayoutTestsscrollingcoordinatorscrollingtreesiblingnodeorderhtml"></a>
<div class="addfile"><h4>Added: trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order.html (0 => 265327)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order.html                            (rev 0)
+++ trunk/LayoutTests/scrollingcoordinator/scrolling-tree/sibling-node-order.html       2020-08-06 14:55:55 UTC (rev 265327)
</span><span class="lines">@@ -0,0 +1,49 @@
</span><ins>+<!DOCTYPE html>
+<html>
+<head>
+    <style>
+        .container {
+            position: relative;
+            z-index: 0;
+            width: 200px;
+            height: 200px;
+            border: 1px solid green;
+        }
+
+        .fixed {
+            position: fixed;
+            left: 0;
+            width: 100%;
+            height: 100px;
+            background-color: rgba(0, 0, 0, 0.2);
+        }
+        .top {
+            top: 0;
+        }
+        .bottom {
+            bottom: 0;
+        }
+    </style>
+    <script>
+        if (window.testRunner)
+            testRunner.dumpAsText();
+        window.addEventListener('load', () => {
+            if (window.internals)
+                document.getElementById('scrollingtree').textContent = internals.scrollingStateTreeAsText();
+            
+        }, false);
+    </script>
+</head>
+<body>
+    <div class="container">
+        <div class="top fixed">
+            Top
+        </div>
+    </div>
+
+    <div class="bottom fixed">
+        Bottom
+    </div>
+<pre id="scrollingtree"></pre>
+</body>
+</html>
</ins></span></pre></div>
<a id="trunkSourceWebCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/ChangeLog (265326 => 265327)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/ChangeLog   2020-08-06 14:48:07 UTC (rev 265326)
+++ trunk/Source/WebCore/ChangeLog      2020-08-06 14:55:55 UTC (rev 265327)
</span><span class="lines">@@ -1,3 +1,39 @@
</span><ins>+2020-08-06  Simon Fraser  <simon.fraser@apple.com>
+
+        Scrolling tree nodes sometimes don't match layer z-order
+        https://bugs.webkit.org/show_bug.cgi?id=215210
+
+        Reviewed by Antti Koivisto.
+
+        When adding nodes to the scrolling state tree during compositing layer traversal,
+        we would sometimes add then in the wrong order. For example, if the composited layer
+        tree was:
+        
+        + Root
+          + Stacking context
+            Fixed layer 1
+          Fixed layer 2
+          
+        we would build a scrolling tree like:
+        
+        + Root scrolling node
+          Node for layer 2
+          Node for layer 1
+
+        This happened because RenderLayerCompositor::updateBackingAndHierarchy() failed to propagate up
+        scrollingTreeState.nextChildIndex.
+        
+        This is generally benign, but inserting node 1 followed by node 2 would be seen as a scrolling tree
+        mutation, causing us to re-commit the scrolling tree when it hadn't really changed, triggering
+        extra CPU usage.
+        
+        Part of <rdar://problem/62737868>: higher CPU usage on instagram.com.
+
+        Test: scrollingcoordinator/scrolling-tree/sibling-node-order.html
+
+        * rendering/RenderLayerCompositor.cpp:
+        (WebCore::RenderLayerCompositor::updateBackingAndHierarchy):
+
</ins><span class="cx"> 2020-08-05  Rob Buis  <rbuis@igalia.com>
</span><span class="cx"> 
</span><span class="cx">         Remove the StyleResolver-specific evaluate function in MediaQueryEvaluator
</span></span></pre></div>
<a id="trunkSourceWebCorerenderingRenderLayerCompositorcpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp (265326 => 265327)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp 2020-08-06 14:48:07 UTC (rev 265326)
+++ trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp    2020-08-06 14:55:55 UTC (rev 265327)
</span><span class="lines">@@ -1356,6 +1356,9 @@
</span><span class="cx"> 
</span><span class="cx">         // Pass needSynchronousScrollingReasonsUpdate back up.
</span><span class="cx">         scrollingTreeState.needSynchronousScrollingReasonsUpdate |= scrollingStateForDescendants.needSynchronousScrollingReasonsUpdate;
</span><ins>+        if (scrollingTreeState.parentNodeID == scrollingStateForDescendants.parentNodeID)
+            scrollingTreeState.nextChildIndex = scrollingStateForDescendants.nextChildIndex;
+
</ins><span class="cx">     } else if (requiresChildRebuild)
</span><span class="cx">         appendForegroundLayerIfNecessary();
</span><span class="cx"> 
</span></span></pre>
</div>
</div>

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