Correct overflow propagation in BTT and RTL writing-modes

Source/core/rendering/RenderBox.cpp

Index: Source/core/rendering/RenderBox.cpp
diff --git a/Source/core/rendering/RenderBox.cpp b/Source/core/rendering/RenderBox.cpp
index 5d3ad3c3fb10e61ee7366b4d10910c57d9fc7dfc..2b94258e97065edae4233c4a11f7048c14d6a5f2 100644
--- a/Source/core/rendering/RenderBox.cpp
+++ b/Source/core/rendering/RenderBox.cpp
@@ -4158,7 +4158,7 @@ void RenderBox::addOverflowFromChild(RenderBox* child, const LayoutSize& delta)
 
 void RenderBox::addLayoutOverflow(const LayoutRect& rect)
 {
-    LayoutRect clientBox = clientBoxRect();
+    LayoutRect clientBox = noOverflowRect();
     if (clientBox.contains(rect) || rect.isEmpty())
         return;
     
@@ -4213,7 +4213,7 @@ void RenderBox::addVisualOverflow(const LayoutRect& rect)
         return;
         
     if (!m_overflow)
-        m_overflow = adoptPtr(new RenderOverflow(clientBoxRect(), borderBox));
+        m_overflow = adoptPtr(new RenderOverflow(noOverflowRect(), borderBox));
     
     m_overflow->addVisualOverflow(rect);
 }
@@ -4407,6 +4407,26 @@ LayoutRect RenderBox::layoutOverflowRectForPropagation(RenderStyle* parentStyle)
     return rect;
 }
 
+LayoutRect RenderBox::noOverflowRect() const
+{
+    // Because of the special coodinate system used for overflow rectangles (not quite logical, not
+    // quite physical), we need to flip the block progression coordinate in vertical-rl and
+    // horizontal-bt writing modes. Apart from that, this method does the same as clientBoxRect().
+
+    LayoutUnit left = borderLeft();
+    LayoutUnit top = borderTop();
+    LayoutUnit right = borderRight();
+    LayoutUnit bottom = borderBottom();
+    // Calculate physical padding box.

[Julien - ping for review, 2013/07/15 23:50:33]

I don't think this comment adds much.

[mstensho (USE GERRIT), 2013/07/17 13:39:36]

Done.

+    LayoutRect rect(left, top, width() - left - right, height() - top - bottom);

[Julien - ping for review, 2013/07/15 23:50:33]

This is very much:

LayoutRect physicalBorderBoxRect = this->borderBoxRect();
physicalBorderBoxRect.move(borderLeft(), borderTop());

(Note: I have no clue why borderBoxRect() doesn't start at (borderLeft(),
borderTop()) apart that it really looks like a bug).

[jbroman, 2013/07/16 01:19:05]

Comment-on-a-comment: the border box is the rect that surrounds the border, not
the rect surrounded by the border. This rect is the rect just within the border.

[mstensho (USE GERRIT), 2013/07/17 13:39:36]

A good method to use here would be paddingBoxRect(), but there are some
complications with scrollbars, due to
https://bugs.webkit.org/show_bug.cgi?id=76129 , so I think we're better off
building the rectangle on our own for now. See new patch regarding the
scrollbars; I've tried to explain it better.

+    // Flip block progression axis if writing mode is vertical-rl or horizontal-bt.

[Julien - ping for review, 2013/07/15 23:50:33]

Same comment about not adding much (it repeats what the code is doing). The
other comments are great though.

[mstensho (USE GERRIT), 2013/07/17 13:39:36]

Done.

+    flipForWritingMode(rect);
+    // Subtract space occupied by scrollbars. They are at their physical edge in this coordinate

[mstensho (USE GERRIT), 2013/07/17 13:39:36]

Raising an issue myself. :)
I didn't understand this comment myself, so I took a closer look at what's going
on (which is why it took me so long to follow up on the review). What we do here
is strange, but it's all due to https://bugs.webkit.org/show_bug.cgi?id=76129

+    // system, so order is important here: first flip, then subtract scrollbars.
+    rect.contract(verticalScrollbarWidth(), horizontalScrollbarHeight());
+    return rect;
+}
+
 LayoutRect RenderBox::overflowRectForPaintRejection() const
 {
     LayoutRect overflowRect = visualOverflowRect();