[Webkit-unassigned] [Bug 214634] New: Fullscreen HTML5 video subtitles/captions are shown twice

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=214634

            Bug ID: 214634
           Summary: Fullscreen HTML5 video subtitles/captions are shown
                    twice
           Product: WebKit
           Version: Safari 13
          Hardware: iPhone / iPad
                OS: iOS 13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Media
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: simonkawaji at gmail.com

When an HTML5 video is played with subtitles/captions in fullscreen, the previous subtitles will shown an additional time for a fraction of a second when the text is changed to the next one. The issue is a bit difficult to describe but you can take a look at the following examples to get a better understanding of the problem.

* Two examples that have the issue:
https://www.iandevlin.com/html5test/webvtt/html5-video-webvtt-sample.html
https://codepen.io/JediLin/full/jpzgpv

* How to recreate the bug:
Try one of these links and fullscreen the video and enable subtitles. 
The bug will occur on fullscreen but not on windowed/in-page mode.

I am using an 5th generation iPad with iOS version 13.6, Safari version 13.1

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