Support for multiple block fragments in getClientRects().

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

Index: third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.cpp
diff --git a/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.cpp b/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.cpp
index b992f1ba0e910c7ba857d00ef63beb29217b8327..0a8a79a27f780c814a2a59f43de81095763912ca 100644
--- a/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.cpp
+++ b/third_party/WebKit/Source/core/layout/MultiColumnFragmentainerGroup.cpp
@@ -437,7 +437,14 @@ void MultiColumnFragmentainerGroup::columnIntervalForBlockRangeInFlowThread(Layo
     logicalBottomInFlowThread = std::min(logicalBottomInFlowThread, this->logicalBottomInFlowThread());
     ASSERT(logicalTopInFlowThread <= logicalBottomInFlowThread);
     firstColumn = columnIndexAtOffset(logicalTopInFlowThread, LayoutBox::AssociateWithLatterPage);
-    lastColumn = columnIndexAtOffset(logicalBottomInFlowThread, LayoutBox::AssociateWithFormerPage);
+    if (logicalBottomInFlowThread == logicalTopInFlowThread) {
+        // Zero-height block range. There'll be one column in the interval. Set it right away. This
+        // is important if we're at a column boundary, since calling columnIndexAtOffset() with the
+        // end-exclusive bottom offset would actually give us the *previous* column.
+        lastColumn = firstColumn;

[mstensho (USE GERRIT), 2016/09/28 19:58:55]

There's something fishy about this whole PageBoundaryRule business. I don't like
to do what I'm doing here.

[eae, 2016/09/28 20:55:17]

Yeah, this smells bad. Would you mind filing a bug about it and adding a link
here?

[mstensho (USE GERRIT), 2016/09/28 21:20:23]

Good idea! crbug.com/651213

+    } else {
+        lastColumn = columnIndexAtOffset(logicalBottomInFlowThread, LayoutBox::AssociateWithFormerPage);
+    }
 }
 
 void MultiColumnFragmentainerGroup::columnIntervalForVisualRect(const LayoutRect& rect, unsigned& firstColumn, unsigned& lastColumn) const