Created attachment 18331 [details]
See the test case and screenshot. Somehow if I draw a filled rectangle and a border around it, a diagonal line appears in that rectangle. But it happens only with certain rectangle sizes and if the left and bottom borders are outside of the window (the test case does it right).
Created attachment 18332 [details]
Compile with gcc `pkg-config --cflags --libs gtk+-2.0`
I've pushed a fix in commit 444d44acde4ac73641c789c19ec7a3199211b594, which will be wrapped up in the forthcoming 1.8 release.