[Webkit-unassigned] [Bug 207256] New: visibilitychange event does not get always fired on background but it may on foreground

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=207256

            Bug ID: 207256
           Summary: visibilitychange event does not get always fired on
                    background but it may on foreground
           Product: WebKit
           Version: Safari 12
          Hardware: iPhone / iPad
                OS: iOS 12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore JavaScript
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: cerutti.alexander at gmail.com

Hello,
this is my first bug here and I'm not too sure about the information I filled to report this problem, since I'm not that into Webkit.

What I experienced is that I've tried to attach a `visibilitychange` listener on my application. It contains a `console.log` and some logics that depend on `document.hidden`.
So, if home button or sleep button is pressed, the event is fired and we can see in the debugging console the log.

I've been reported, and I've verified, that sometimes this event is not fired when, for sure on iPad (also iOS 13), one of those two buttons are pressed. Instead the event is fired when Safari goes back in foreground. So the log gets printed twice. Moreover, sometimes `document.hidden` is `false` in both events. So if some logic is based on a check on that property, wrong code block may be run.

I wasn't able to find a steady way to reproduce this, and this seems to happen in different cases.
For example, I'm developing a video player (here is where the event is created). We are importing a third-party video AD engine. What we experienced is that during some ADs, on home/sleep buttons, the event is not fired on background but twice on foreground. This happened also during other video contents playback, but I'm not sure it is related to multimedia contents playback but instead on the amount of work the engine that is has to do when the buttons get pressed or some kind of concurrency problem on Safari background (but these are only thoughts of mine).

If I'll ever find more details I'll edit or add a comment to increase the details.
I hope this bug report is good enough (but I think this won't be) to find the problem.
Thank you.

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