Rework mapToVisualRectInAncestorSpace to handle flipped blocks correctly.

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
@@ skipping 10 matching lines @@
                     origin
                 |<-------->|
                  _____________________
                 |          |          |
                 |          |          |
                 |          |          |
 direction:rtl   |          |   box    |
                 |          |          |
                 |          |          |
                 |__________|__________|
-                

[chrishtr, 2016/07/11 22:35:11]

Spurious edits in this file.

[wkorman, 2016/07/11 22:46:17]

Done.

+
                       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
                     |<--------------------->|
+
+## Coordinate Spaces
+
+TODO(wkorman): Document writing mode, particularly flipped blocks.