A proper workaround for this problem is applying this CSS rule: div.RadEditor .RadRibbonBar .rrbButton img.rrbButtonImage { top:-7px; }
The issue is not reproducible with Lightweight render mode.
I also found that the AddComment button is partially cut off when used in the RibbonBar. When small or medium, the left side is cut off. When large, the right side is cut off.