This is still present in 6.3.76760 but it seems to be a GTK issue.
I have attached Ian's test case (which doesn't involve Eiffel at all) in test.c
so that this can be tracked.  I reproduced the memory leak (heap keeps growing)
on Solaris 10 SPARC, using GTK+2.14.4 and other very recent packages.

I suggest submitting a GTK bug report with this test case.  Hopefully, they will
either fix it or say what is wrong with the program.