I've noticed that when playing back videos (1920x1080) larger than my laptop screen (1366x768), mpv ends up defaulting to 1920x1080, however; Window Display → Screen Limits `Keep windows within the screen visual limits` is set. mpv doesn't do this when I test the same video from Cinnamon, the entire window stays within the screen.
Also when this happens in E, sending input to mpv becomes somewhat difficult. `f` to full screen doesn't work, and double clicking the window takes more than double clicking.