[Webkit-unassigned] [Bug 254374] New: REGRESSION (260451 at main): Opening any PDF in WebKit opens it halfway down the first page

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=254374

            Bug ID: 254374
           Summary: REGRESSION (260451 at main): Opening any PDF in WebKit
                    opens it halfway down the first page
           Product: WebKit
           Version: Other
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: PDF
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: tyler_w at apple.com
                CC: thorton at apple.com

Before 260451 at main, the sequence for updating PDF scroll position on and shortly after load was this:

1. PDF plugin is initialized, scroll position is set to (0, 0)
2. PDFKit does -[PDFLayerController _updateAutoScale] and -[PDFLayerController magnifyWithMagnification:atPoint:immediately:], and informs the WKPDFLayerControllerDelegate to update the scroll position to some greater than zero value (despite no scroll actually happening)
3. A call to PDFPlugin::receivedNonLinearizedPDFSentinel() is handled. Scroll position is set to (0, 0)

After 260451 at main, the sequence became:

1. PDF plugin is initialized, scroll position is set to (0, 0)
2. PDFKit does magnification as described above, updates scroll position to (0, 190 (or other >0 value)). This is dispatched to the main run loop to be handled asynchronously.
3. A call to PDFPlugin::receivedNonLinearizedPDFSentinel() is handled. Scroll position is set to (0, 0)
4. The async dispatch from step 2 is handled by the main-runloop, overwriting the (0,0) value

rdar://106880773

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