[Explorer] has a drawing glitch when used on XPSP3 with Luna theme and vertically enlarged taskbar

I used the ros explorer from 0.4.15-dev-3408-g8d10682 here.

When we use that in XPSP3 and do use the MS Luna theme, and expand the taskbar in y, we do see a drawing glitch. Something is not optimal within our explorer binary.

xpsp3_MSLuna_MSexplorer.webm
xpsp3_MSLuna_ros_explorer_unpatched.webm

Just to prove that PR4121 will not help here, it looks the same as unpatched:
xpsp3_MSLuna_ros_explorer_PR4121patched.webm

And in classic mode XPSP3 explorer and ros explorer do look the same, we are fine there. (no drawing dirt in classic mode, when using the ros explorer)