fix #24735: allocate space for fretboard diagrams

Remaining (not new!) issue:
For staves with fretboards the chord symbols' edit box jumps up (too high, final output) and down (correct position, but just during input), in a similar fashion to befor fix #24735 and they are still showing up too high (by 9sp on 2nd staff, 16.25sp on 3rd 22.75 on 4th, etc.)