Using current trunk, I can reproduce this using dholbert's steps. With my patch for bug #297080 (which is Martijn's nsEventStateManager.cpp patch plus gtk2 widget support for generating mouse exits based on his Win32 code) applied, the problem does not occur. So, yeah, basically confirming what Sylvain said in comment #19.
Using current trunk, I can reproduce this using dholbert's steps. With my patch for bug #297080 (which is Martijn's nsEventStateMan ager.cpp patch plus gtk2 widget support for generating mouse exits based on his Win32 code) applied, the problem does not occur. So, yeah, basically confirming what Sylvain said in comment #19.