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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/281913">281913</a></dd>
<dt>Author</dt> <dd>commit-queue@webkit.org</dd>
<dt>Date</dt> <dd>2021-09-02 02:11:58 -0700 (Thu, 02 Sep 2021)</dd>
</dl>

<h3>Log Message</h3>
<pre>Absolutely positioned and negative z-index div with canvas child gets drawn with wrong stacking order
https://bugs.webkit.org/show_bug.cgi?id=217154

Patch by Rob Buis <rbuis@igalia.com> on 2021-09-02
Reviewed by Simon Fraser.

Source/WebCore:

Keep overlap map accurate.

Test: compositing/subtree-div-overlaps-multiple-negative-z-divs.html

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

LayoutTests:

Add test based on bug report test case.

* compositing/subtree-div-overlaps-multiple-negative-z-divs-expected.html: Added.
* compositing/subtree-div-overlaps-multiple-negative-z-divs.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="#trunkLayoutTestscompositingsubtreedivoverlapsmultiplenegativezdivsexpectedhtml">trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs-expected.html</a></li>
<li><a href="#trunkLayoutTestscompositingsubtreedivoverlapsmultiplenegativezdivshtml">trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs.html</a></li>
</ul>

</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkLayoutTestsChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/LayoutTests/ChangeLog (281912 => 281913)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/ChangeLog      2021-09-02 08:52:06 UTC (rev 281912)
+++ trunk/LayoutTests/ChangeLog 2021-09-02 09:11:58 UTC (rev 281913)
</span><span class="lines">@@ -1,3 +1,15 @@
</span><ins>+2021-09-02  Rob Buis  <rbuis@igalia.com>
+
+        Absolutely positioned and negative z-index div with canvas child gets drawn with wrong stacking order
+        https://bugs.webkit.org/show_bug.cgi?id=217154
+
+        Reviewed by Simon Fraser.
+
+        Add test based on bug report test case.
+
+        * compositing/subtree-div-overlaps-multiple-negative-z-divs-expected.html: Added.
+        * compositing/subtree-div-overlaps-multiple-negative-z-divs.html: Added.
+
</ins><span class="cx"> 2021-09-01  Sonia Singla  <ssingla@igalia.com>
</span><span class="cx"> 
</span><span class="cx">         Replace webkit- prefix logical properties with Standard Properties in LayoutTests/
</span></span></pre></div>
<a id="trunkLayoutTestscompositingsubtreedivoverlapsmultiplenegativezdivsexpectedhtml"></a>
<div class="addfile"><h4>Added: trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs-expected.html (0 => 281913)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs-expected.html                                (rev 0)
+++ trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs-expected.html   2021-09-02 09:11:58 UTC (rev 281913)
</span><span class="lines">@@ -0,0 +1,2 @@
</span><ins>+<!DOCTYPE html>
+<div style="background:green;width:100px;height:100px"></div>
</ins></span></pre></div>
<a id="trunkLayoutTestscompositingsubtreedivoverlapsmultiplenegativezdivshtml"></a>
<div class="addfile"><h4>Added: trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs.html (0 => 281913)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs.html                         (rev 0)
+++ trunk/LayoutTests/compositing/subtree-div-overlaps-multiple-negative-z-divs.html    2021-09-02 09:11:58 UTC (rev 281913)
</span><span class="lines">@@ -0,0 +1,27 @@
</span><ins>+<!DOCTYPE html>
+<style>
+.negative {
+  z-index: -1;
+}
+.abs {
+  position: absolute;
+}
+.green {
+  width: 100px;
+  height: 100px;
+  background-color: green;
+}
+
+.red {
+  width: 100px;
+  height: 100px;
+  background-color: red;
+  will-change: transform;
+}
+</style>
+<body>
+<div class="negative abs">
+  <div class="red"></div>
+</div>
+<div class="negative abs green"></div>
+</body>
</ins></span></pre></div>
<a id="trunkSourceWebCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/ChangeLog (281912 => 281913)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/ChangeLog   2021-09-02 08:52:06 UTC (rev 281912)
+++ trunk/Source/WebCore/ChangeLog      2021-09-02 09:11:58 UTC (rev 281913)
</span><span class="lines">@@ -1,3 +1,17 @@
</span><ins>+2021-09-02  Rob Buis  <rbuis@igalia.com>
+
+        Absolutely positioned and negative z-index div with canvas child gets drawn with wrong stacking order
+        https://bugs.webkit.org/show_bug.cgi?id=217154
+
+        Reviewed by Simon Fraser.
+
+        Keep overlap map accurate.
+
+        Test: compositing/subtree-div-overlaps-multiple-negative-z-divs.html
+
+        * rendering/RenderLayerCompositor.cpp:
+        (WebCore::RenderLayerCompositor::computeCompositingRequirements):
+
</ins><span class="cx"> 2021-09-02  Mark Lam  <mark.lam@apple.com>
</span><span class="cx"> 
</span><span class="cx">         Add more support for JIT operation validation testing.
</span></span></pre></div>
<a id="trunkSourceWebCorerenderingRenderLayerCompositorcpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp (281912 => 281913)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp 2021-09-02 08:52:06 UTC (rev 281912)
+++ trunk/Source/WebCore/rendering/RenderLayerCompositor.cpp    2021-09-02 09:11:58 UTC (rev 281913)
</span><span class="lines">@@ -1098,16 +1098,19 @@
</span><span class="cx">     bool anyDescendantHas3DTransform = false;
</span><span class="cx">     bool descendantsAddedToOverlap = currentState.hasNonRootCompositedAncestor();
</span><span class="cx"> 
</span><ins>+    unsigned newlyCompositedChildLayerCount = 0;
</ins><span class="cx">     for (auto* childLayer : layer.negativeZOrderLayers()) {
</span><span class="cx">         computeCompositingRequirements(&layer, *childLayer, overlapMap, currentState, backingSharingState, anyDescendantHas3DTransform);
</span><span class="cx"> 
</span><span class="cx">         // If we have to make a layer for this child, make one now so we can have a contents layer
</span><span class="cx">         // (since we need to ensure that the -ve z-order child renders underneath our contents).
</span><del>-        if (!willBeComposited && currentState.subtreeIsCompositing) {
-            layer.setIndirectCompositingReason(IndirectCompositingReason::BackgroundLayer);
-            layerWillComposite();
-        }
</del><ins>+        if (!willBeComposited && currentState.subtreeIsCompositing)
+            ++newlyCompositedChildLayerCount;
</ins><span class="cx">     }
</span><ins>+    while (newlyCompositedChildLayerCount--) {
+        layer.setIndirectCompositingReason(IndirectCompositingReason::BackgroundLayer);
+        layerWillComposite();
+    }
</ins><span class="cx"> 
</span><span class="cx">     for (auto* childLayer : layer.normalFlowLayers())
</span><span class="cx">         computeCompositingRequirements(&layer, *childLayer, overlapMap, currentState, backingSharingState, anyDescendantHas3DTransform);
</span></span></pre>
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