[REGEDIT] redraw on splitter move shows gray area

This was tested on ReactOS version 0.4.15-dev-7778-gc6b8638. See the videos.

Reproduction steps:
1) Open regedit and get a window with the splitter near the center with some values on the right side.
2) Grab the splitter and move it nearly all the way to the right.
3) Grab the splitter and move it back to the center.

Note: This issue will SOMETIMES not appear.
There is absolutely NOTHING logged to the debug log.

Patch attached regedit_splitter_fix.patch fixes this for me.