[Webkit-unassigned] [Bug 202696] New: WKWebView occasionally fails to render content when scrolling

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=202696

            Bug ID: 202696
           Summary: WKWebView occasionally fails to render content when
                    scrolling
           Product: WebKit
           Version: Safari 12
          Hardware: iPhone / iPad
                OS: iOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: teodor.atroshenko at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Created attachment 380434

  --> https://bugs.webkit.org/attachment.cgi?id=380434&action=review

iPhone 5s screenshot

An app is using WKWebView for main screen. Since the end of August, users are experiencing a random bug where after launching or after returning from background, they see no content (just a dark gray area) below certain position on the page (when scrolling). This position is not constant - sometimes it's right at the top of the page (still below the first screen, on iPhone X it is around 444 points below the bottom of the screen [aka at around 1256 points from top of the page]), other times it is further down the page (in cases when it happens after returning from background). When the bug happens, scrolling is available up and down (works correctly). But no JavaScript seems to get executed: no click/touch events are fired and remote debugging (via Safari Preview + USB cable) is partially broken - connection is established (Sources are populated), however console is non-functional. I've gotten reports that this resolves itself after about 30 seconds, but typically users just restart the app and don't wait for it to fix itself. I've attached a screenshot to illustrate what the user sees when it happens.

Possibly related to bug 193665. However, I'd guess its the JS engine getting stuck and taking the renderer/compositor with it. There is no crash so no crash log, but I'm open to suggestions about how I can gather more data on this.

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