[Webkit-unassigned] [Bug 251229] New: Mastodon video exits fullscreen immediately in Safari, MiniBrowser assertion failure

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=251229

            Bug ID: 251229
           Summary: Mastodon video exits fullscreen immediately in Safari,
                    MiniBrowser assertion failure
           Product: WebKit
           Version: WebKit Local Build
          Hardware: Mac (Apple Silicon)
                OS: macOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Media
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: opendarwin at lapcatsoftware.com

Created attachment 464676

  --> https://bugs.webkit.org/attachment.cgi?id=464676&action=review

MiniBrowser lldb backtrace

Summary:
There's a video on Mastodon that causes Safari to exit fullscreen as soon as you enter fullscreen. This doesn't happen in Firefox or Google Chrome.

I've tested with Safari 16.3 and Safari Technology Preview Release 162 running macOS Ventura 13.2 and Monterey 12.6.3 on M1 Apple silicon.

I also built WebKit from source (commit 8c0586df02759523c7ba36188adcb4f85a2033de) and tested with MiniBrowser, which hits an assertion failure after the video exits fullscreen. Attached is the lldb backtrace.

Steps to reproduce:
1) Open https://mstdn.plus/@lapcatsoftware
2) Scroll down to the video from https://mstdn.plus/@lapcatsoftware/10974137414728390 on January 23. (Note that the bug doesn't reproduce if you open the post directly, for unknown reasons. You have to use the timeline from step 1.)
3) Open the contextual menu on the video and select Enter Full Screen.

Expected results: The video plays in full screen.

Actual results: The video flashes goes into full screen momentarily, flashes, and exits full screen.

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