According to Andrew Ellis:
I'm trying to use Profont, but if I type an f followed by an i, the spacing goes crazy, ie. the letters move close together, but the cursor
I don't have this problem but using a non standard font (Bitstream Vera Mono, a very nice fixed free font), underscore have a tendency to disappear. Scrolling seems to get some back then other disappear again.
See the following screenshot, I believe it is quite easy to spot where underscore should be :)
http://www.keltia.net/download/no_underscore.pdf
Thanks for TM by the way, although I see some things missing (preferences menu for example), TM seems a blessing after struggling to find a decent editor (although jEdit comes close, it is Java).