[Webkit-unassigned] [Bug 97365] New: ScrollingTreeState needs to be a tree of nodes

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=97365

           Summary: ScrollingTreeState needs to be a tree of nodes
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: bdakin at apple.com


This is a step toward getting pages with fixed position to be able to be scrolled by the scrolling thread. In order to do that, fixed layers will have to be represented by nodes in the ScrollingTree…currently, the scrolling tree only ever has one node, so we have to do a little code restructuring to get it into a state where is can expect and handle multiple nodes. This is a first step along that path.

This patch specifically looks at ScrollingTreeState. Right now, the ScrollingTreeState attempts to contain all of the state information needed for the whole scrolling tree in one object. But in the future when there are multiple nodes in the scrolling tree, a single state object will not be sufficient. ScrollingState should also be represented by a tree. Technically, a HashTable would work too, but a tree will be better suited to communicating parent/child/sibling relationships for the scroll layers over to the ScrollingTree. This patch makes scrolling state into a tree. The old ScrollingTreeState class has become the ScrollingStateScrollingNode class since the majority of the class represents scroll state that is specific to ScrollableAreas and will not be applicable to fixed or sticky layers.

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