I found the same issue. Might it be the screen DPI scaling in Windows 10?
I deleted my .ini, open tdl which creates a new .ini with defaults, Maximize tdl window, and then changed the SplitPos by a few pixels (325->328) and noticed the thumb icon for the slider shifted over. So I moved it more (360) so that it was not visible. When I mouse over where it should be, the mouse cursor changes for resize but I'm not able to resize. Capture attached. I just do a quick ctrl-shift-m twice to get things back in order.
Doesn't happen unless I maximize.
[FileStates\Introduction.tdl]
HiddenExtensionCount=0
ListViewVisible=1
ShowAttributes=1
SplitPos=360
View=-1