[New Multicolumn] Don't enter child multicol contexts when processing object insertions / removals.

Source/core/layout/LayoutMultiColumnFlowThread.cpp

Index: Source/core/layout/LayoutMultiColumnFlowThread.cpp
diff --git a/Source/core/layout/LayoutMultiColumnFlowThread.cpp b/Source/core/layout/LayoutMultiColumnFlowThread.cpp
index 80850ec8c1f9e36dbf369fcceb8744162b1e462a..219fa2c1559e0dc97f630d6363ab0a9d633261cb 100644
--- a/Source/core/layout/LayoutMultiColumnFlowThread.cpp
+++ b/Source/core/layout/LayoutMultiColumnFlowThread.cpp
@@ -445,27 +445,49 @@ LayoutUnit LayoutMultiColumnFlowThread::skipColumnSpanner(LayoutBox* renderer, L
     return adjustment;
 }
 
+// When processing layout objects to remove or when processing layout objects that have just been
+// inserted, certain types of objects should be skipped.
+static bool shouldSkipInsertedOrRemovedChild(const LayoutObject& child)
+{
+    if (child.isLayoutFlowThread()) {
+        // Found an inner flow thread. We need to skip it and its descendants.
+        return true;
+    }
+    if (child.isLayoutMultiColumnSet() || child.isLayoutMultiColumnSpannerPlaceholder()) {
+        // Column sets and spanner placeholders in a child multicol context don't affect the parent
+        // flow thread.
+        return true;
+    }
+    return false;
+}
+
 void LayoutMultiColumnFlowThread::flowThreadDescendantWasInserted(LayoutObject* descendant)
 {
     ASSERT(!m_isBeingEvacuated);
-    LayoutObject* nextRenderer = descendant->nextInPreOrderAfterChildren(this);
+    LayoutObject* objectAfterSubtree = descendant->nextInPreOrderAfterChildren(this);
     // This method ensures that the list of column sets and spanner placeholders reflects the
     // multicol content after having inserted a descendant (or descendant subtree). See the header
     // file for more information. Go through the subtree that was just inserted and create column
     // sets (needed by regular column content) and spanner placeholders (one needed by each spanner)
     // where needed.
-    for (LayoutObject* renderer = descendant; renderer; renderer = renderer->nextInPreOrder(descendant)) {
+    LayoutObject* next;
+    for (LayoutObject* renderer = descendant; renderer; renderer = next) {

[dsinclair, 2015/04/24 13:13:30]

nit: s/renderer/layoutObject/

[mstensho (USE GERRIT), 2015/04/24 13:27:26]

Done.

Note that I didn't really introduce this in this CL, though.

+        if (shouldSkipInsertedOrRemovedChild(*renderer)) {
+            next = renderer->nextInPreOrderAfterChildren(descendant);
+            continue;
+        }
+        next = renderer->nextInPreOrder(descendant);
         if (containingColumnSpannerPlaceholder(renderer))
             continue; // Inside a column spanner. Nothing to do, then.
         if (descendantIsValidColumnSpanner(renderer)) {
             // This renderer is a spanner, so it needs to establish a spanner placeholder.
             LayoutBox* insertBefore = 0;
             LayoutMultiColumnSet* setToSplit = 0;
-            if (nextRenderer) {
+            if (objectAfterSubtree) {
                 // The spanner is inserted before something. Figure out what this entails. If the
                 // next renderer is a spanner too, it means that we can simply insert a new spanner
                 // placeholder in front of its placeholder.
-                insertBefore = nextRenderer->spannerPlaceholder();
+                insertBefore = objectAfterSubtree->spannerPlaceholder();
                 if (!insertBefore) {
                     // The next renderer isn't a spanner; it's regular column content. Examine what
                     // comes right before us in the flow thread, then.
@@ -483,7 +505,7 @@ void LayoutMultiColumnFlowThread::flowThreadDescendantWasInserted(LayoutObject*
                         // set this is. It needs to be split it into two sets, so that we can insert
                         // a new spanner placeholder between them.
                         setToSplit = findSetRendering(previousRenderer);
-                        ASSERT(setToSplit == findSetRendering(nextRenderer));
+                        ASSERT(setToSplit == findSetRendering(objectAfterSubtree));
                         setToSplit->setNeedsLayoutAndFullPaintInvalidation(LayoutInvalidationReason::ColumnsChanged);
                         insertBefore = setToSplit->nextSiblingMultiColumnBox();
                         // We've found out which set that needs to be split. Now proceed to
@@ -498,17 +520,17 @@ void LayoutMultiColumnFlowThread::flowThreadDescendantWasInserted(LayoutObject*
             continue;
         }
         // This renderer is regular column content (i.e. not a spanner). Create a set if necessary.
-        if (nextRenderer) {
-            if (LayoutMultiColumnSpannerPlaceholder* placeholder = nextRenderer->spannerPlaceholder()) {
+        if (objectAfterSubtree) {
+            if (LayoutMultiColumnSpannerPlaceholder* placeholder = objectAfterSubtree->spannerPlaceholder()) {
                 // If inserted right before a spanner, we need to make sure that there's a set for us there.
                 LayoutBox* previous = placeholder->previousSiblingMultiColumnBox();
                 if (!previous || !previous->isLayoutMultiColumnSet())
                     createAndInsertMultiColumnSet(placeholder);
             } else {
-                // Otherwise, since |nextRenderer| isn't a spanner, it has to mean that there's
+                // Otherwise, since |objectAfterSubtree| isn't a spanner, it has to mean that there's
                 // already a set for that content. We can use it for this renderer too.
-                ASSERT(findSetRendering(nextRenderer));
-                ASSERT(findSetRendering(renderer) == findSetRendering(nextRenderer));
+                ASSERT(findSetRendering(objectAfterSubtree));
+                ASSERT(findSetRendering(renderer) == findSetRendering(objectAfterSubtree));
             }
         } else {
             // Inserting at the end. Then we just need to make sure that there's a column set at the end.
@@ -528,9 +550,15 @@ void LayoutMultiColumnFlowThread::flowThreadDescendantWillBeRemoved(LayoutObject
     if (m_isBeingEvacuated)
         return;
     bool hadContainingPlaceholder = containingColumnSpannerPlaceholder(descendant);
+    bool processedSomething = false;
     LayoutObject* next;
     // Remove spanner placeholders that are no longer needed, and merge column sets around them.
     for (LayoutObject* renderer = descendant; renderer; renderer = next) {
+        if (shouldSkipInsertedOrRemovedChild(*renderer)) {
+            next = renderer->nextInPreOrderAfterChildren(descendant);
+            continue;
+        }
+        processedSomething = true;
         LayoutMultiColumnSpannerPlaceholder* placeholder = renderer->spannerPlaceholder();
         if (!placeholder) {
             next = renderer->nextInPreOrder(descendant);
@@ -549,8 +577,8 @@ void LayoutMultiColumnFlowThread::flowThreadDescendantWillBeRemoved(LayoutObject
         }
         placeholder->destroy();
     }
-    if (hadContainingPlaceholder)
-        return; // We're only removing a spanner (or something inside one), which means that no column content will be removed.
+    if (hadContainingPlaceholder || !processedSomething)
+        return; // No column content will be removed, so we can stop here.
 
     // Column content will be removed. Does this mean that we should destroy a column set?
     LayoutMultiColumnSpannerPlaceholder* adjacentPreviousSpannerPlaceholder = 0;