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

third_party/WebKit/Source/core/layout/README.md

+# `Source/core/layout`
+
+## Overflow rects and scroll offsets
+
+PaintLayerScrollableArea uses a "scroll origin" to conceptually represent the distance between
+the top-left corner of the box'es content rect and the top-left corner of its overflow rect
+when the box is scrolled to the logical beginning of its content (.e.g. all the way to the left for
+LTR, all the way to the right for RTL).  For left-to-right and top-to-bottom flows, the scroll
+origin is zero, i.e., the top/left of the overflow rect is at the same position as the top/left of
+the box'es content rect when scrolled to the beginning of flow.  For right-to-left and bottom-to-top
+flows, the overflow rect extends to the top/left of the client rect.
+
+The default calculation for scroll origin is the distance between the top-left corner of the content
+rect and the top-left corner of the overflow rect.  To illustrate, here is a box with left overflow and
+no vertical scrollbar:
+
+                             content
+                               rect
+                           |<-------->|
+                    scroll
+                    origin
+                |<-------->|
+                 _____________________
+                |          |          |
+                |          |          |
+                |          |          |
+direction:rtl   |          |   box    |
+                |          |          |
+                |          |          |
+                |__________|__________|
+                
+                      overflow rect
+                |<--------------------->|
+
+
+However, if the box has a scrollbar for the orthogonal direction (e.g., a vertical scrollbar
+in a direction:rtl block), the size of the scrollbar must be added to the scroll origin calculation.
+Here are two examples -- note that it doesn't matter whether the vertical scrollbar is placed on
+the right or left of the box (the vertical scrollbar is the |/| part):
+
+                               content
+                                 rect
+                             |<-------->|
+                    scroll
+                    origin
+                |<---------->|
+                 _______________________
+                |          |/|          |
+                |          |/|          |
+                |          |/|          |
+direction:rtl   |          |/|   box    |
+                |          |/|          |
+                |          |/|          |
+                |__________|/|__________|
+
+                      overflow rect
+                |<--------------------->|
+                
+                
+                
+                                 content
+                                   rect
+                               |<-------->|
+                      scroll
+                      origin
+                  |<---------->|
+                     _______________________
+                    |          |          |/|
+                    |          |          |/|
+                    |          |          |/|
+writing-mode:       |          |          |/|
+  vertical-rl       |          |          |/|
+                    |          |          |/|
+                    |          |          |/|
+                    |          |          |/|
+                    |__________|__________|/|
+                    
+                          overflow rect
+                    |<--------------------->|