Scrollbars theming regression

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.