libreoffice overwrite cell confirmation dialog - graphics corruption

After I leave libreoffice calc open for a few hours, when I try to paste in a cell that already contains data, I get a confirmation dialog and the "yes" button is corrupted.
Sometimes a single character in the cell data will display corrupted.
It may be related.
How do I narrow down the problem?
http://lockie.ca/libreoffice//libreoffice_confirm.jpg