First of all sorry for my bad English.
This is a bug which happens from EVER, inclusive in UmPlayer, really I can't imagina why never fixed this bug in years.
When you have SMplayer/UmPlayer minimized and you load another video, the jump keys (short jump, middle jump, large jump) stop working.
Steps to reproduce this bug:
Load a video into smplayer/umplayer.
Minimize the UI to the taskbar.
Double click on another video to load into smplayer/umplayer.
At this point you can't use the jump keys.