[Webkit-unassigned] [Bug 75568] New: Lazily allocate overflow: hidden layers if we have overflowing content

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=75568

           Summary: Lazily allocate overflow: hidden layers if we have
                    overflowing content
           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: jchaffraix at webkit.org
                CC: hyatt at apple.com, simon.fraser at apple.com,
                    dglazkov at chromium.org, jamesr at chromium.org
            Blocks: 73714


Related to bug 75001 and 73715.

We currently allocate our RenderLayer's whenever the style change in RenderBoxModel::styleDidChange. However for some values of 'overflow', we actually don't need to allocate the layer until we have some layout overflow (to hold the scroll position, paint the scrollbars, ...). This bug makes the lazy allocation work for overflow: hidden layers (the easiest case). It could be extended for 'overflow: auto' in a follow up bug.

The upside of the change is better performance for elements that can work without a layer (the RenderLayer code path can be a lot slower that the normal RenderObject one) and less memory use. We need to add some NULL-checks in the code but it is a fairly low number. Unfortunately this change also lead to a massive change of our text baselines.

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