This bug was reported on Windows 10, but I can reproduce this "mouse pointer won't disappear" problem on YouTube and Netflix on macOS when my mouse pointer is moved all the way to the right edge of the screen. However, when the mouse pointer is hovering over the middle of the video, it disappears after a couple seconds as expected. Enabling the `full-screen-api.macos-native-full-screen` pref in bug 1631735 seems to fix the problem on macOS for me.
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This bug was reported on Windows 10, but I can reproduce this "mouse pointer won't disappear" problem on YouTube and Netflix on macOS when my mouse pointer is moved all the way to the right edge of the screen. However, when the mouse pointer is hovering over the middle of the video, it disappears after a couple seconds as expected.