The video player has many hotkeys, most usefully Spacebar to pause/play and M to mute/unmute. When an extension is enabled on a stream(it may only be some extensions, but I've noticed this with the vast majority), it seems to form an invisible pane that will eat clicks on the player (i.e. highlighting it to use the hotkeys). Hotkeys still work if you can select the player, which you can check by clicking the control strip at the bottom.

This doesn't really feel like intended behavior and is extremely confusing if you don't know about it.