The measured character width is way too high in Linux, causing the cursor to be out of sync with the text and making the editor unusable.
The problem was that charWidth didn't specify the font before measuring it. It just so happened that Linux usually has different font widths for the gutter and the editor. Fixed by adding a getCharacterWidth() method to the canvas mixin that requires the font to be specified. [needs-pull] Changeset f9dd8928a9c9 @ http://bitbucket.org/pcwalton/bespinclient/
This is on tip.
Since this bug has been resolved, I am clearing the whiteboard and removing the 'need-pull' tag. If you feel this was in error, feel free to put it back. (However, please provide an explanation for doing so.) [This was a mass comment. To filter bugmail, use the term 'bespin-remove-needs-pull'.]