I remember old versions of wine emulated rich text boxes with a plain text box - this hasn't been the case for a while AFAIK, so it might be worth a shot if you haven't tried it in a few months.
Edit: I just gave TextPad a quick try, and it seems to work correctly. The default colors seemed kind of messed up, but it was changeable in the preferences.
At risk of starting an editor flamewar... it only comes with syntax highlighting for 4 languages??? Kate has highlighting for 10 _categories_ of file types, 37 in 'sources'. Sadly though, it doesn't have highlighting for ARM assembly (has MIPS though, which might be easily modifiable).