Version: 14.3.0 from RPMFusion on Fedora.
When SMPlayer is maximized at the time the fullscreen action is invoked, it will unmaximize the window before switching into fullscreen mode. This causes glitchy appearance on window managers which animate window size changes, such as KWin with default settings. The reverse happens when switching out of fullscreen mode, with similar consequences.
Here's an upstream ticket of this being discussed: https://bugs.kde.org/show_bug.cgi?id=336467