Fix scroll origin, overflow rects, and coordinate flipping for flexbox.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 12f489202c52d6b429840b7777bd6fb20455b4a5..06d8fea21ad6e59ffb86fbca2ff2044ae4c4a6e4 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -587,14 +587,49 @@ int PaintLayerScrollableArea::pixelSnappedScrollHeight() const
     return snapSizeToPixel(scrollHeight(), box().clientTop() + box().location().y());
 }
 
+IntSize PaintLayerScrollableArea::originAdjustmentForScrollbars() const

[leviw_travelin_and_unemployed, 2016/04/28 22:12:12]

This deserves a big description about why it exists and what the cases are. I
know that's a bit of a pain to write up, but I don't want the arcane knowledge
you acquired to just be cargo-culted forever :)

+{
+    IntSize size;
+    int verticalScrollbarWidth = box().verticalScrollbarWidth();
+    int horizontalScrollbarHeight = box().horizontalScrollbarHeight();
+
+    if (box().hasFlippedBlocksWritingMode()) {
+        if (box().isFlexibleBox()) {

[leviw_travelin_and_unemployed, 2016/04/28 22:12:12]

It seems like it may be easier/clearer to handle the FlexBox case first and
separately (maybe even in another function that describes why it's different),
then the  non-flex case should seem easier.

The two intertwined like this makes it harder to understand the different cases.

+            EFlexDirection flexDirection = box().style()->flexDirection();
+            if (flexDirection != FlowColumnReverse)
+                size.expand(verticalScrollbarWidth, 0);
+            if (flexDirection == FlowRowReverse)
+                size.expand(0, horizontalScrollbarHeight);
+        } else {
+            size.expand(verticalScrollbarWidth, 0);
+        }
+    } else if (box().isHorizontalWritingMode()) {
+        if (box().isFlexibleBox()) {
+            EFlexDirection flexDirection = box().style()->flexDirection();
+            if (flexDirection == FlowRowReverse)
+                size.expand(verticalScrollbarWidth, 0);
+            else if (flexDirection == FlowColumnReverse)
+                size.expand(0, horizontalScrollbarHeight);
+        } else if (box().shouldPlaceBlockDirectionScrollbarOnLogicalLeft()) {
+            size.expand(verticalScrollbarWidth, 0);
+        }
+    } else if (box().isFlexibleBox()) {
+        EFlexDirection flexDirection = box().style()->flexDirection();
+        if (flexDirection == FlowColumnReverse)
+            size.expand(verticalScrollbarWidth, 0);
+        else if (flexDirection == FlowRowReverse)
+            size.expand(0, horizontalScrollbarHeight);
+    }
+    return size;
+}
+
 void PaintLayerScrollableArea::computeScrollDimensions()
 {
     m_overflowRect = box().layoutOverflowRect();
     box().flipForWritingMode(m_overflowRect);
 
-    int scrollableLeftOverflow = m_overflowRect.x() - box().borderLeft() - (box().shouldPlaceBlockDirectionScrollbarOnLogicalLeft() ? box().verticalScrollbarWidth() : 0);
-    int scrollableTopOverflow = m_overflowRect.y() - box().borderTop();
-    setScrollOrigin(IntPoint(-scrollableLeftOverflow, -scrollableTopOverflow));
+    LayoutPoint scrollableOverflow = m_overflowRect.location() - LayoutSize(box().borderLeft(), box().borderTop());
+    setScrollOrigin(flooredIntPoint(-scrollableOverflow) + originAdjustmentForScrollbars());
 }
 
 void PaintLayerScrollableArea::scrollToPosition(const DoublePoint& scrollPosition, ScrollOffsetClamping clamp, ScrollBehavior scrollBehavior, ScrollType scrollType)