Scrollbars theming regression, sometimes a themed scrollbar is not drawn until you hover it with the mouse

Reproducing steps:

1. Enable any theme in ReactOS besides Classic (e. g. Lunar).
2. Open My Computer.
3. Go to C:\ReactOS or C:\ReactOS\system32 directory.
4. Scroll the folder up/down via mouse wheel or via left mouse button.
5. Look at the scrollbars.

0.4.14-dev-1133-g6831468ccfdedb4eb8c7cc88ac2a01f35dbfc16b - they draw correctly. Theming doesn't disappear.
0.4.14-dev-1134-g00adb1a3f967ac7f5cd56b4c39df72a7b3814603 - they draw incorrectly. Theming disappears very often. When scrolling the folder via mouse wheel, it's completely missing on the scrollbar itself whole time, but not on the scrollbar arrows. If to scroll via left mouse button, theming disappears intermittently, when moving the scrollbar itself. Up and Down arrows almost are not affected.

It can be reproduced by any other way, via any program, which uses themed scrollbars (builtin or native). E. g., I tested Task Manager, and there are exactly the same sympthoms with the scrollbars.

The guilty commit seems to be improvements in the scollbars drawing code in our Win32SS. So I guess this is a source of actual problem, if not uxtheme.

Demonstration videos and debug.log are also attached. Btw, I don't see any debug spam in the log from win32ss/user/ntuser/scrollbar.c.

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Edit by reactosfanboy: I just realized, that we did not even have a video in here of any affected build, so I decided to attach a video that demonstrates on the last affected build, that
the themed scrollbars are broken and sometimes not properly drawn (interacting with the vertical scrollbar breaks the horizontal one and the other way round). The other scrollbar is not properly painted unless it is hovered again with the mouse pointer
0.4.15-dev-3085-g515e0cf_ThisVideoDemonstrates_CORE-16735_whatIsWrongWithThemes_ButHowItWorksFineWithClassic.webm
but the video does also show that the unthemed scrollbars do work fine in that build