Clear box from percentage descendants when it is split off from the ancestor

third_party/WebKit/Source/core/layout/LayoutBox.cpp

Index: third_party/WebKit/Source/core/layout/LayoutBox.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBox.cpp b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
index fb82a7c0afc2369dcb56c705915c16ed387d0d39..060a7b55bdb7c8a9bc330b6607d1987a45e84944 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBox.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBox.cpp
@@ -4670,6 +4670,23 @@ static void markBoxForRelayoutAfterSplit(LayoutBox* box)
     box->setNeedsLayoutAndPrefWidthsRecalcAndFullPaintInvalidation(LayoutInvalidationReason::AnonymousBlockChange);
 }
 
+static bool collapseIfWillBecomeLoneAnonymousBlock(LayoutBox* child)
+{
+    if (!child->isAnonymousBlock())
+        return false;
+    LayoutBlock* childBlock = toLayoutBlock(child);
+    if (childBlock->beingDestroyed())
+        return false;
+    // We're splitting from our previousSibling().

[mstensho (USE GERRIT), 2016/01/21 19:12:33]

It's not lone if it has a previousSibling(). Please add a test, or at least an
assertion that previousSibling() is NULL.

[rhogan, 2016/01/24 12:31:37]

No, but it's about to split from the previousSibling so it will become a lone
anonymous block. Test 2 covers this.

[mstensho (USE GERRIT), 2016/01/25 09:17:07]

That's making an assumption about what the caller is doing. If we want to do
that, maybe skip the childBlock->beingDestroyed() check as well? And maybe just
put the code inside splitAnonymousBoxesAroundChild(), since we're making scary
assumptions anyway.

+    if (child->nextSibling() || childBlock->continuation())
+        return false;
+    LayoutBlock* parent = child->containingBlock();
+    if (parent->isRubyBase() || parent->isRubyRun())
+        return false;
+    LayoutBlock::collapseAnonymousBlockChild(parent, childBlock);
+    return true;
+}
+
 LayoutObject* LayoutBox::splitAnonymousBoxesAroundChild(LayoutObject* beforeChild)
 {
     bool didSplitParentAnonymousBoxes = false;
@@ -4694,9 +4711,17 @@ LayoutObject* LayoutBox::splitAnonymousBoxesAroundChild(LayoutObject* beforeChil
             markBoxForRelayoutAfterSplit(boxToSplit);
             markBoxForRelayoutAfterSplit(postBox);
 
+            // Splitting the box means the left side of the container chain will lose any percent height descendants
+            // below |postBox| in the right hand side.
+            if (LayoutBlock::hasPercentHeightContainerMap())
+                LayoutBlock::clearPercentHeightDescendantsFrom(postBox);
+
             beforeChild = postBox;
         } else {
-            beforeChild = boxToSplit;
+            // If a lone anonymous block is now unnecessary collapse it away - this will make beforeChild's current
+            // grandparent its parent so no need to set it before looping.

[mstensho (USE GERRIT), 2016/01/21 19:12:33]

Thanks for explaining!

+            if (!collapseIfWillBecomeLoneAnonymousBlock(boxToSplit))
+                beforeChild = boxToSplit;
         }
     }