I don’t know what exactly it was I typed to change it, but the seek bar in fullscreen became a very basic one:
The normal seek bar had the normal media player controls, such as volume, play, pause and so on:
It took me quite a few attempts via trial and error to identify the correct setting.
To re-enable the normal seek bar in fullscreen mode, click View » Options, select the Video tab and unmark Exclusive fullscreen (ringed in red):
Click OK to save and apply the settings change and the old seek bar will be back.