WIP: Construct SPV2's scroll paint property tree

third_party/WebKit/Source/platform/graphics/paint/README.md

 # Platform paint code
 
 This directory contains the implementation of display lists and display
 list-based painting, except for code which requires knowledge of `core/`
 concepts, such as DOM elements and layout objects.
 
 This code is owned by the [paint team][paint-team-site].
 
 Slimming Paint v2 is currently being implemented. Unlike Slimming Paint v1, SPv2
 represents its paint artifact not as a flat display list, but as a list of
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 Each clip node has:
 
 * A float rect with (optionally) rounded corner radius.
 * An associated transform node, which the clip rect is based on.
 
 The raster region defined by a node is the rounded rect transformed to the
 root space, intersects with the raster region defined by its parent clip node
 (if not root).
 
+### Scrolling
+
+Each paint chunk is associated with a [scroll node](ScrollPaintPropertyNode.h)
+which defines information about how a subtree scrolls so threads other than the
+main thread can perform scrolling. Scroll information includes:
+
+* Which directions, if any, are scrollable by the user.
+* A reference to a [transform node](TransformPaintPropertyNode.h) which contains
+a 2d scroll offset.
+* The extent that can be scrolled. For example, an overflow clip of size 7x9
+with scrolling contents of size 7x13 can scroll 4px vertically and none
+horizontally.
+
+To ensure geometry operations are simple and only deal with transforms, the
+scroll offset is stored as a 2d transform in the transform tree.
+
 ### Effects
 
 Each paint chunk is associated with an [effect node](EffectPaintPropertyNode.h),
 which defines the effect (opacity, transfer mode, filter, mask, etc.) that
 should be applied to the content before or as it is composited into the content
 below.
 
 Each effect node has:
 
 * a floating-point opacity (from 0 to 1, inclusive)
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 visual and transformed rects of display items in the coordinate space of ancestor
 [`PropertyTreeState`](PropertyTreeState.h)s.
 
 The transformed rect of a display item in an ancestor `PropertyTreeState` is that
 rect, multiplied by the transforms between the display item's `PropertyTreeState`
 and the ancestors, then flattened into 2D.
 
 The visual rect of a display item in an ancestor `PropertyTreeState` is the intersection
 of all of the intermediate clips (transformed in to the ancestor state), with
 the display item's transformed rect.