When using gVim snapped to one of the corners of my Windows 8.1 desktop (Windows Key + Left / Right key), if I invoke NERDTreeToggle command, the gVim window unsnaps from the corner and redraws itself near the centre of the screen.
I am having to snap gVim window back into it's corner every time I use NERDTreeToggle command.
This happens when gvim has scrollbars turned on. Check your :set guioptions?
. If you see L or R, then you need to remove them.
:set guioptions-=L
:set guioptions-=R