[Webkit-unassigned] [Bug 234066] New: blob videos slow to pause

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=234066

            Bug ID: 234066
           Summary: blob videos slow to pause
           Product: WebKit
           Version: Safari 15
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Media
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: opendarwin at lapcatsoftware.com

Created attachment 446491

  --> https://bugs.webkit.org/attachment.cgi?id=446491&action=review

screenshot of web inspector breakpoint

This bug affects some HTML <video> elements with a blob: URL src. When JavaScript pause() is called on such a video soon after play(), the video can be very slow to pause, taking several seconds.

I'm not sure when exactly the bug was introduced, but I think it was in some version of Safari 15. I can reproduce the bug on iPadOS 15.1, as well as on macOS 11.6.1 (Intel) and macOS 12.0.1 (Apple Silicon) with Safari 15.1 and Safari Technology Preview 136. It's easier to give steps to reproduce on Mac, so I'll do that.

Steps to reproduce:
1. In Safari Preferences, Websites, set Auto-Play to Allow All Auto-Play.
2. Open the web inspector to the Sources tab.
3. Add an event breakpoint for the play event.
4. Add the following breakpoint condition:

event.target instanceof HTMLVideoElement && event.target.src.startsWith('blob:') && !event.target.paused

5. Add the following breakpoint action:

event.target.pause(); console.log('pause ' + event.target.src);

6. Set the breakpoint option to Automatically continue after evaluating.
7. Open https://detroit.cbslocal.com/2021/12/03/prosecutor-lays-out-disturbing-timeline-explaining-why-oxford-school-shooting-suspects-parents-were-charged/
8. Wait for the video to load (you can watch the Console).

The bug might not happen every time, so you may have to reload. The bug also occurs with https://www.cnn.com/videos/world/2021/12/08/allegra-stratton-boris-johnson-downing-street-party-video-ctw-intl-ldn-vpx.cnn (and many other CNN videos), but you'll have to sit through the ad to wait for the main video.

An alternative for reproducing on macOS and iPadOS would be to create a Safari extension to do the same thing as the breakpoint, i.e., addEventListener for play.

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