I get the same thing on most of my Ubuntu systems. Strangely one works without a problem. Exact same packages installed. Go figure. I agree it's probably something to do with the GUI library. I am able to work around this issue by highlighting text then typing over it. So say a box reads "1" and you want to change it to "2". Normally you'd place the cursor after the 1, press Backspace, then prby superjamie - Slic3r