The playback buttons in picture-in-picture mode get in the way, making it impossible to read the subtitles. And while these buttons may hide, reappear when you hover over the video window, or when you restart or stop playback, you could relocate the buttons to some other area. But it would be great to relocate them outside the window floating.
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The reason PiP controls overlap with the subtitles is because these subtitles are "burnt into" the video (and we cannot detect or control their display). If you go to youtube and activate the subtitles, then launch PiP, you will see that they will never overlap with controls.
I understand the inconvenience when using the burn-in subs is very real, but moving controls outside of the window to improve the situation is not something that will likely make it to our roadmap (as they represent a fraction of all subtitled content).