A fix for RT-13914 introduced a new boolean field in GlassView2D and GlassView3D objects: lockedFocus. It gets updated from the View.begin()/end() methods, and access to the field isn't synchronized currently. Since all known Glass clients invoke both begin() and end() on the same thread, this shouldn't cause any issues.

However, if we want to keep Glass thread-safe, we need to synchronize access to this field.