Doh! Sorry, was looking at the top of the picture! Yeah, cleartype I reckon. Doesn't happen here.
Put into tracker:
Mmh, obviously I've fixed this by myself. Registry setting of "MS Shell Dlg" to "Arial Unicode MS" gives this result (bold active docker tabs). But it's not done by a default Reaper installation.
Color swap do the job, no bold font needed...its ugly