Comment 6 for bug 1036096

'Assertion "r == n_visible_rows" failed' in GTK, that good ol' GTK bug :)

Anyway it's not Geany's fault, and a workaround for such GTK bugs was introduced in Geany 0.21 (released 2011-10-02). See upstream reports:
https://sourceforge.net/tracker/?func=detail&aid=3311258&group_id=153444&atid=787791
https://sourceforge.net/tracker/?func=detail&aid=3304273&group_id=153444&atid=787791
https://sourceforge.net/tracker/?func=detail&aid=3201050&group_id=153444&atid=787791
https://sourceforge.net/tracker/?func=detail&aid=3163742&group_id=153444&atid=787791
https://sourceforge.net/tracker/?func=detail&aid=3153120&group_id=153444&atid=787791
https://sourceforge.net/tracker/?func=detail&aid=2985896&group_id=153444&atid=787791