Clip region based columns properly.

Source/core/rendering/RenderMultiColumnSet.cpp

 /*
  * Copyright (C) 2012 Apple Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 264 matching lines @@
     // In the block direction, we will not clip overflow out of the top of the first column, or out of the bottom of
     // the last column. This applies only to the true first column and last column across all column sets.
     //
     // FIXME: Eventually we will know overflow on a per-column basis, but we can't do this until we have a painting
     // mode that understands not to paint contents from a previous column in the overflow area of a following column.
     // This problem applies to regions and pages as well and is not unique to columns.
     bool isFirstColumn = !index;
     bool isLastColumn = index == colCount - 1;
     bool isLeftmostColumn = style()->isLeftToRightDirection() ? isFirstColumn : isLastColumn;
     bool isRightmostColumn = style()->isLeftToRightDirection() ? isLastColumn : isFirstColumn;
-    LayoutRect overflowRect(portionRect);
+
+    // Calculate the overflow rectangle, based on the flow thread's, clipped at column logical
+    // top/bottom unless it's the first/last column.
+    LayoutRect overflowRect = overflowRectForFlowThreadPortion(portionRect, isFirstColumn && isFirstRegion(), isLastColumn && isLastRegion());
+
+    // Avoid overflowing into neighboring columns, by clipping in the middle of adjacent column
+    // gaps. Also make sure that we avoid rounding errors.
     if (isHorizontalWritingMode()) {
-        if (isLeftmostColumn) {
-            // Shift to the logical left overflow of the flow thread to make sure it's all covered.
-            overflowRect.shiftXEdgeTo(min(flowThread()->visualOverflowRect().x(), portionRect.x()));
-        } else {
-            // Expand into half of the logical left column gap.
+        if (!isLeftmostColumn)
             overflowRect.shiftXEdgeTo(portionRect.x() - colGap / 2);
-        }
-        if (isRightmostColumn) {
-            // Shift to the logical right overflow of the flow thread to ensure content can spill out of the column.
-            overflowRect.shiftMaxXEdgeTo(max(flowThread()->visualOverflowRect().maxX(), portionRect.maxX()));
-        } else {
-            // Expand into half of the logical right column gap.
-            overflowRect.shiftMaxXEdgeTo(portionRect.maxX() + colGap / 2);
-        }
+        if (!isRightmostColumn)
+            overflowRect.shiftMaxXEdgeTo(portionRect.maxX() + colGap - colGap / 2);
     } else {
-        if (isLeftmostColumn) {
-            // Shift to the logical left overflow of the flow thread to make sure it's all covered.
-            overflowRect.shiftYEdgeTo(min(flowThread()->visualOverflowRect().y(), portionRect.y()));
-        } else {
-            // Expand into half of the logical left column gap.
+        if (!isLeftmostColumn)
             overflowRect.shiftYEdgeTo(portionRect.y() - colGap / 2);
-        }
-        if (isRightmostColumn) {
-            // Shift to the logical right overflow of the flow thread to ensure content can spill out of the column.
-            overflowRect.shiftMaxYEdgeTo(max(flowThread()->visualOverflowRect().maxY(), portionRect.maxY()));
-        } else {
-            // Expand into half of the logical right column gap.
-            overflowRect.shiftMaxYEdgeTo(portionRect.maxY() + colGap / 2);
-        }
+        if (!isRightmostColumn)
+            overflowRect.shiftMaxYEdgeTo(portionRect.maxY() + colGap - colGap / 2);
     }
-    return overflowRectForFlowThreadPortion(overflowRect, isFirstRegion() && isFirstColumn, isLastRegion() && isLastColumn);
+    return overflowRect;
 }
 
 void RenderMultiColumnSet::paintObject(PaintInfo& paintInfo, const LayoutPoint& paintOffset)
 {
     if (style()->visibility() != VISIBLE)
         return;
 
     RenderBlock::paintObject(paintInfo, paintOffset);
 
     // FIXME: Right now we're only painting in the foreground phase.
@@ skipping 180 matching lines @@
         fragments.append(fragment);
     }
 }
 
 const char* RenderMultiColumnSet::renderName() const
 {
     return "RenderMultiColumnSet";
 }
 
 }