Themed scrollbars, the most rightmost pixels of the horizontal scrollbar is not drawn initially

A continuation of CORE-16735. A minor drawing thingy is left here as it seems.

Reproduction steps

• activate the Lunar theme
• run Notepad by clicking the readme.txt on desktop
• watch the rightmost pixels of the horizontal scrollbar

Expected result
They should be drawn initially

Observed result
They are not drawn initially unless we hover the horizontal scrollbar at least once with the mouse
0.4.14-RC-99-g970fa88_affected.webm (this was a private build just, that I created just on my local PC, but that I will not commit. It had https://github.com/reactos/reactos/commit/236649c626f97c68a9d0a6709a1bb342681b2aa0 integrated in it. I will not commit this into the RC, as it still has too many issues)
0.4.14-RC-99-g970fa88_affected.log (nothing is logged while hovering)
The same bug happens with the bottom-most pixels of the vertical scrollbar