Use float for scroll offset.

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

 # `Source/core/layout`
 
 This directory contains implementation of layout objects. It covers the
 following document lifecycle states:
 
 * LayoutSubtreeChange (`InLayoutSubtreeChange` and `LayoutSubtreeChangeClean`)
 * PreLayout (`InPreLayout`)
 * PerformLayout (`InPerformLayout`)
 * AfterPerformLayout (`AfterPerformLayout` and `LayoutClean`)
 
 Note that a new Blink layout system is under development. See the
 [LayoutNG design document](https://docs.google.com/document/d/1uxbDh4uONFQOiGuiumlJBLGgO4KDWB8ZEkp7Rd47fw4/preview).
 
-## Overflow rects and scroll offsets
+## Overflow and scrolling
 
-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.
+When a LayoutBox has scrollable overflow, it is associated with a PaintLayerScrollableArea.
+PaintLayerScrollableArea uses a "scroll origin" to represent the location of the top/left
+corner of the content rect (the visible part of the content) in the coordinate system
+defined by the top/left corner of the overflow rect, when the box is scrolled all the way
+to the beginning of its content.
 
-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:
+For content which flows left-to-right and top-to-bottom, the scroll origin will be (0, 0),
+i.e., the top/left of the content rect is coincident with the top/left of the overflow rect
+when the box is scrolled all the way to the beginning of content.
 
-                                 content
-                                   rect
-                               |<-------->|
-                        scroll
-                        origin
-                    |<-------->|
-                     _____________________
-                    |          |          |
-                    |          |          |
-                    |          |          |
-    direction:rtl   |          |   box    |
-                    |          |          |
-                    |          |          |
-                    |__________|__________|
+For content which flows right-to-left (including direction:ltr, writing-mode:vertical-rl,
+and flex-direction:row-reverse), the x-coordinate of the scroll origin will be positive;
+and for content which flows bottom-to-top (e.g., flex-direction:column-reverse and
+vertical writing-mode with direction:ltr), the y-coordinate of the scroll origin will be
+positive.
 
-                          overflow rect
-                    |<--------------------->|
+In all cases, the term 'scrollOffset' (or just 'offset') is used to represent the distance
+of the scrolling viewport from its location when scrolled to the beginning of content, and
+it uses type ScrollOffset. The term 'scrollPosition' (or just 'position') represents a
+point in the coordinate space defined by the overflow rect, and it uses type FloatPoint.
 
+For illustrations of these concepts, see these files:
 
-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):
+doc/ltr-tb-scroll.png
+doc/rtl-bt-scroll.png
+doc/rtl-tb-scroll.png
+
+Unfortunately, the geometric abstraction of origin, offset, and position breaks down slighlty

[skobes, 2016/10/06 20:06:53]

"slightly"

[szager1, 2016/10/06 21:07:03]

Done.

+when computing the scroll origin 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       |          |          |/|
-                        |          |          |/|
-                        |          |          |/|
-                        |          |          |/|
-                        |__________|__________|/|
+                                 content
+                                   rect
+                               |<-------->|
+                        scroll
+                        origin
+                    |----------->|
+                     _______________________

[skobes, 2016/10/06 20:06:53]

I still can't wrap my head around this picture.

Why does the overflow rect overlap the right-side scrollbar?  If this is the
start-of-flow position the right edge of the content area should be visible,
with the scrollbar extruding to the right.  Then the content rect would be
aligned with the scroll origin just like it is in the left-side scrollbar
diagram.

[szager1, 2016/10/06 21:07:03]

The overflow rect is determined independently of whether the container has a
vertical scrollbar.  So, its dimensions are the same whether or not there's a
vertical scrollbar.  This is why we need
LayoutBox::originAdjustmentForScrollbars.

The downside is that the mental model breaks down: the overflow rect extends to
the 15 pixels underneath the vertical scrollbar.  The upside is that scrollbar
existence (especially overflow:auto scrollbar is existence) is volatile, but we
avoid ever mucking with the location of the overflow rect once it has been
established during layout.

+                    |          |          |/|
+                    |          |          |/|
+                    |          |          |/|
+    writing-mode:   |          |    box   |/|
+      vertical-rl   |          |          |/|
+                    |          |          |/|
+                    |__________|__________|/|
+    
+                          overflow rect
+                    |<--------------------->|
 
-                              overflow rect
-                        |<--------------------->|
 
 ## Coordinate Spaces
 
 Layout and Paint work with and frequently refer to three main coordinate spaces
 (really two, with one variant):
 
 * Physical coordinates: Corresponds to physical direction of the output per the
   physical display (screen, printed page). Generally used for painting, thus
   layout logic that feeds into paint may produce values in this space. CSS
   properties such as `top`, `right`, `bottom`, and `left` are in this space. See
@@ skipping 200 matching lines @@
   left-to-right or right-to-left block flow. Geometry is transposed for vertical
   writing mode. See calls to `transposed{Rect,Point,Size}()`.
 * `direction`/`dir`: "inline base direction" of a box. One of `ltr` or
   `rtl`. See calls to `isLeftToRightDirection()`.
 * `text-orientation`: orientation of text in a line. Only relevant for vertical
   modes.
 * orthogonal flow: when a box has a writing mode perpendicular to its containing
   block. This can lead to complex cases. See
   [specification](https://www.w3.org/TR/css-writing-modes-3/#orthogonal-flows)
   for more.