Comment 6 for bug 772972

Yes bededog is right. A good workaround is to edit /usr/share/themes/Ambiance/metacity-1/metacity-theme-1.xml

Just need to change the padding of frame_style to be equal values for name="normal_unfocused" and name="normal_focused". Then change theme to something else and back again to Ambiance to enable the change.

While it looks like a mistake in the Ambiance theme that these padding values are different, it's not really the root cause of the bug. The root cause is a code issue because this invisible padding area is not meant to affect the visible size of the window. You should be able to have any values for padding and the window size should not change magically by itself.