I use geany 0.18 (from ubuntu 9.10)
I have simple shell script.
Most lines are shorter than 80 characters, but I have some bit longer.
Exactly: 6079 chars length.
When I set cursor at end of this line - view is broken.
When I move cursor to another line or begin of this line - view is ok.
Would it be possible for you to try updating and reproducing this issue with a more current version of Geany? 0.18 got quiet old in the meantime. Maybe install from the Ubuntu PPA at https://launchpad.net/~geany-dev/+archive/ppa. I guess 0.20 will also be available there soon.