Huge gap in staves when switching from page to horizontal continuous view

I like to work on scores in the horizontal continuous view but am finding that if I close one of the parts down and then reopen it I end up with a huge gap between the parts.

If I close it down and reopen it rights but then as soon as I close then reopen a part it happens again!

Opening and closing parts within a score is also very very slow to react.

I worked on this for quite a while a week ago and it was fine so it seems to be something new.

I have the most up to date version of the software.

Any ideas? It's driving me mad!