This issue is not new to v18 but now is a great time to get it fixed.
Steps to reproduce (on Windows):
1) Start SlickEdit 'clean' (i.e. empty config folder)
2) Select 'Finish' to get past the preliminaries
3) Auto-hide the left and bottom tool window groups
4) Open an empty document (e.g. <ESC> new-file <ENTER>)
5) Select View->Full Screen
See attached screenshot of the bottom left corner of the SlickEdit screen. There exists a 'square' that should not be...
Consistently reproducible for me on both WinXP (SE 17.0.3) and Win7 x64 (SE 17.0.3 and SE v18 Beta).