Sadly, that does not tell us much. The power manager does the screen dimming, but it should not in this scenario.
At this point I can only suggest to open a report on the Xfce bug tracker. https://bugzilla.xfce.org/ ** Changed in: xfce4-power-manager (Ubuntu) Status: Incomplete => New -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1772325 Title: Screen dims while a video is playing in VLC To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xfce4-power-manager/+bug/1772325/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs