[Webkit-unassigned] [Bug 84965] New: [chromium] Composited fixed-position elements are wrongly positioned when scroll position is restored from history

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=84965

           Summary: [chromium] Composited fixed-position elements are
                    wrongly positioned when scroll position is restored
                    from history
           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: husky at google.com


Forked off from https://bugs.webkit.org/show_bug.cgi?id=70103 because it's a bug in existing code.

Steps to reproduce:
- Open any page
- Open another page with a composited fixed-position element (e.g. one with a 3D transform)
- Scroll down
- Go back to the previous page
- Go forward

The scroll position is restored, but the fixed-position element will be in the wrong place. When you scroll further it pops back to the correct position.

I think this is because the scroll position is restored while we're inside FrameView::layout(), and updateFixedElementsAfterScrolling() looks like this:

  void FrameView::updateFixedElementsAfterScrolling()
  {
  #if USE(ACCELERATED_COMPOSITING)
      if (!m_nestedLayoutCount && hasFixedObjects()) {
          if (RenderView* root = rootRenderer(this)) {
              root->compositor()->updateCompositingLayers(CompositingUpdateOnScroll);
          }
      }
  #endif
  }

m_nestedLayoutCount is 1, so we don't call updateCompositingLayers().

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