Created attachment 478327 [details] Screenshot of the bug Some YouTube videos are cropped on the left and right when they enter fullscreen. Steps to reproduce: 1. Open https://www.youtube.com/watch?v=IIqtuupvdWg in Safari 2. Control-double-click on the video to open the Safari native contextual menu rather than the YouTube custom contextual menu 3. Select Enter Full Screen from the contextual menu Expected results: The video shows the black and white border on the left and right sides. Actual results: The left and right sides are cropped. Compare with the second screenshot of YouTube full screen mode, triggered by the "f" key, which puts a container element into fullscreen rather than just the video. The issue may be specific to retina displays or to certain Mac models. I can reproduce on a 16-inch M4 Pro MacBook Pro but not on an M1 Mac mini with a non-retina display. Another person has confirmed that they can reproduce on a 15-inch M4 MacBook Air.