Running KDE on Kubuntu 13.04 (on two different systems), if you change the font hintstyle to either "none" or "slight", the bottom part of kdiff3 (the merge/edit window) doesn't handle cursor positioning correctly. Clicking the mouse pointer results in a slight offset (to the left) of where the cursor actually ends up. Also, once the cursor is positioned, editing, such as hitting the backspace key, causes the wrong character to be affected. Basically, it looks like the cursor is shown in one spot, but kdiff3 acts like the cursor is a little to the right - about 1 character, at size 9. At larger fonts, you'll notice that the cursor is placed in the middle of a character, instead of before or after the character.
I've tried several fonts that come with Kubuntu, including DejaVu Sans Mono. Putting the hintstyle to medium eliminates the problem. Anti-aliasing doesn't seem to affect this.
I'm curious if anyone else can reproduce this behavior.