bracket behaviour incorrect when some parts are hidden

In MuseScore 2 beta 1 and the current nightly version, there seems to be a problem with putting brackets on staves if some of the instruments are hidden.

1. Create a new score with multiple instruments, e.g. voice, piano, guitar tab, drumset.
2. Use menu Add > Instruments... and hide the guitar and drumset.
3. Drag a bracket over the brace in the piano part top stave. Expect all braces to turn into brackets. Actual behaviour is that only the top one does.
4. Select one of the brackets and delete it.
5. Drag a brace from the palette onto the piano top stave.
6. Double click the brace, and try to resize it to cover both piano staves. Expect all piano parts to be drawn with a brace over both staves. Actual behaviour is that brace being edited jumps upwards to have a negative height, until it is deselected.

This can be mitigated by showing all the instrument parts, setting up the braces as required, then hiding some instrument parts again.