[Webkit-unassigned] [Bug 110294] New: Out-of-view fixed position check should not be affected by page scale at all on Mac

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=110294

           Summary: Out-of-view fixed position check should not be
                    affected by page scale at all on Mac
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Keywords: InRadar
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: bdakin at apple.com


https://bugs.webkit.org/show_bug.cgi?id=104303 and then https://bugs.webkit.org/show_bug.cgi?id=104943 introduced behavior where the out-of-view fixed position logic scales the layer bounds by the page scale factor before comparing it to the visible content rect (which is NOT scaled by the page scale). This is definitely wrong, at least on the Mac. It seems like it is very much intended behavior on Chromium based on comments in the layout tests that were added by that change.

These changes seem to assume that a fixed element that was offscreen might come onscreen when a user scales a page in or out…or that an onscreen element might go offscreen after a page scale in a way that is not accessible by scrolling. This is absolutely not the case on Mac. On Mac, the aim of page scaling is to totally preserve the page as it was…just to make it bigger or smaller. So fixed elements that were onscreen will still be onscreen, and elements that were off will still be off.

I am going to upload a patch that fixes this on the Mac. For the time being, I will assume that the Chromium behavior was intentional…but I do have to say that is strikes me as quite odd; so it would be nice to have some Chromium people confirm that that is indeed their desired behavior. 

<rdar://problem/13247582> Fixed elements disappear on zooming

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