Clicking a disabled scrollbar button should not trigger its redraw

A very subtle issue.
I used Midnight Commander setup from rapps here, but it can be tested with any arbitrary scrollbar that is disabled, most likely.

Expected result
Clicking a disabled scrollbar button should not redraw it

Observed result
Clicking a disabled scrollbar button does trigger a redraw unnecessarily
0.4.14-RC-117-g5e81087_affected.webm (it's almost unnoticable in the video, due to recording frequency, but if you look closely then you can see it in the video as well in the later parts)
0.4.14-RC-117-g5e81087_affected.log (nothing gets logged)
I am very sure that current master head 0.4.15-dev-3338-g0e75fc9 does still behave the same.

Tip:
If you can not see this on your high-end-machine then just artitificially limit the amount of CPU that VBox does spend to your VM, easy to see then.