[trunk] R4797 First clef disappears when double-clicked

1. Create a standard piano score
2. Double-click the first system treble or bass clef symbols. They temporarily disappear
3. Click on a blank portion of the score to exit Edit mode.
4. If you right-click where the now invisible clefs were you can view their properties.
5. Press some undefined key like S or J. The clefs are visible again. Many other operations will also make them visible