Hey there, just reporting an issue I've been having with all the E3 streams so far. That is, the player is acting as though I'm hovering over the window, thus showing the video player controls, progress bar, and slight vignetting at the bottom of the video. So far the only reliable workaround I've found is that if I go into the player settings and reselect the "High" option the controls will disappear until the next time I move the mouse a pixel. It's not the world's worst problem, but it's relatively annoying, especially since the progress bar goes right through all the staff.
I'm on Windows 10 and the most recent version of Firefox (67.0.1). For what it's worth, this behavior does not happen in normal videos, and although I haven't been around for many recent non-E3 livestreams it's never been a problem for me in the past. It also doesn't happen while I'm chatting in GBi, but that doesn't use the native player so that's expected.