[Webkit-unassigned] [Bug 221252] New: Assertion failure in Debug build blocks webpage rendering

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=221252

            Bug ID: 221252
           Summary: Assertion failure in Debug build blocks webpage
                    rendering
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: Minor
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: estea at igalia.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Overview:

Assertion failure from: ASSERT(willBeComposited == needsToBeComposited(layer, queryData)); in rendering/RenderLayerCompositor.cpp stops the rendering of some webpages in MiniBrowser in Debug builds. The reason seems to be that the layers are asynchronously updated since the time willBeComposited is set and needsToBeComposited is called, and so events like scrolling overflow can cause assertion failure. The assertion should be moved right after willBeComposited is set.

Steps to reproduce:
Build WPE/Webkit in Debug mode and run MiniBrowser with https://www.igalia.com. 

Results:
An empty page will be displayed.

Expected Result:
The webpage to be displayed.

Proposed Fix:
If the assertion is moved right after:

    if (layer.needsPostLayoutCompositingUpdate() || compositingState.fullPaintOrderTraversalRequired || compositingState.descendantsRequireCompositingUpdate) {
        layer.setIndirectCompositingReason(IndirectCompositingReason::None);
        willBeComposited = needsToBeComposited(layer, queryData);
    }
the page will appear fine in MiniBrowser.

Additional Information:
Bug was spotted on Linux/X11 using nested Weston and running MiniBrowser like this:
./run-wpe-x11 <path_to_minibrowser_installdir>MiniBrowser https://www.igalia.com

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