User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:57.0) Gecko/20100101 Firefox/57.0 Build ID: 20170905220108 Steps to reproduce: 1. Play a video on either Youtube or Netflix 2. Enter fullscreen Actual results: I Found that there is a cutout on the right side of my screen where the dock lives (it isn't visible though). See attached screenshot. I am seeing this on both 57 and the latest 55 release. It appears to be related to macOS High Sierra in some way. I am on the latest Public Beta. Expected results: It should have played the video in fullscreen without a cutout. The cutout will move if I move where the dock is. Also, if I set the dock to autohide, the issue doesn't occur. Also if I fullscreen Firefox prior to making the video fullscreen, the issue doesn't occur. Finally, I do not see this issue in Safari, Brave, or Chrome.