Monodevelop can not open gtk designer. By trying to open gtk designer mono develop crash. By emerging monodevelop the package metacity will not automatically emerged, but monodevelop need the libmetacity.so.0 to run gtk designer. emerge metacity and rebuild monodevelop will solve the problem.
please, mark this bugreport as a duplicate of the bug: https://bugs.gentoo.org/show_bug.cgi?id=132465
*** This bug has been marked as a duplicate of 132465 ***