It also works fine on OpenSolaris snv_134 on x86.  I don't know
what version of GTK and associated libraries are being used.

This could be a bug in GTK+ but we have been using this same
version for years.  This problem does not occur in 6.5.

Can you give me any hints about what might have changed between
6.5 and 6.6 that could possibly cause this problem?