In the latest plasma version, a line is being displayed at the top of the player when a video is maximized.
I'm reporting here first because this seems to be a smplayer bug, since all the other players that I tested this bug didn't occur, including the mpv itself.
I did the tests on my real machine (archlinux) and on a virtual machine using KDE Neon unstable. Both presented the same problem.
KDE Plasma Version: 5.20.90
KDE Frameworks Version: 5.78.0
Qt Version: 5.15.2
MPV: 0.33.0
SMPlayer: 21.1.0