Never destroy the root content layer unless leaving compositing mode.

third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp

Index: third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
index 58a1ff7a906827e2aca5ae432dd0daef1d512b24..10526bfd209c36c60ca12a016c349ce5f13e3a17 100644
--- a/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/PaintLayerCompositor.cpp
@@ -437,10 +437,10 @@ void PaintLayerCompositor::updateIfNeeded()
             GraphicsLayerTreeBuilder().rebuild(*updateRoot, ancestorInfo);
         }
 
-        if (childList.isEmpty())
-            destroyRootLayer();
-        else if (m_rootContentLayer)
+        if (!childList.isEmpty()) {

[pdr., 2016/06/10 21:07:08]

If we are leaving compositing, could we write:
if (staleInCompositingMode())
    destroyRootLayer()

?

I am worried about leaving around stale root layers. Is this codepath hit in any
layout test? If so, could you use that test to add an assert somewhere that
ensures the root layer is destroyed?

[chrishtr, 2016/06/10 21:20:01]

The bug is that in compositing mode we are destroying. We shouldn't do that.
Yes it will stay around sometimes, but I don't think that will cause any issues.
I will try running
the layout tests with such an assert.

+            CHECK(m_rootContentLayer && m_compositing);
             m_rootContentLayer->setChildren(childList);
+        }
 
         applyOverlayFullscreenVideoAdjustmentIfNeeded();
     }