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<!DOCTYPE html>
<html>
<head>
<style>
#container {
position: absolute;
z-index: 0;
top: 50px;
left: 50px;
width: 300px;
height: 300px;
background-color: gray;
}
#compositedNegZIndex {
position: absolute;
z-index: -5;
width: 100px;
height: 100px;
background-color: blue;
overflow: scroll;
}
#tall {
height: 1000px;
}
#innocentNegZIndex {
position: absolute;
z-index: -2;
top: 200px;
left: 200px;
width: 100px;
height: 100px;
background-color: lime;
}
#explanation {
position: absolute;
top: 400px;
}
</style>
<script>
if (window.internals) {
window.internals.settings.setPreferCompositingToLCDTextEnabled(true);
}
function doTest() {
if (!window.internals)
document.getElementById("explanation").style.display = "block";
if (window.testRunner)
testRunner.dumpAsText();
}
window.addEventListener("load", doTest, false);
</script>
</head>
<body>
<div id="container">
<div id="compositedNegZIndex">
<div id="tall"></div>
</div>
<div id="innocentNegZIndex"> </div>
</div>
<pre id="explanation" style="display:none;">
Crash after hitting below bottom of overlap stack: https://code.google.com/p/chromium/issues/detail?id=285979
The crashing scenario happened as follows:
- force-compositing-mode is disabled. As a result, compositing mode is
disabled until a compositing trigger.
- overflow-scroll as the compositing trigger, because it does not enable
compositing mode before computeCompositingRequirements. Other triggers
happen during CompositedLayerMapping incremental update entry points.
(Note: CompositedLayerMapping was formerly known as LayerBacking.)
- The above two requirements are necessary to create the scenario where
the root layer does not create an overlap context.
- The overflow-scroll element has a negative z-index which causes the
parent layer to be composited, too.
- As a result, subsequent children layers need to add themselves to the
overlap map, even if they are not composited.
- The problem, however, is that we never created an overlap context for
these negative z-index non-composited layers. When the layers try to add
themselves to the overlap, they try to access below the bottom of the
stack, and crash.
</pre>
</body>
</html>
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