Unfortunately it’s not practical for Dorico to adjust the staff spacing automatically in galley view based on the extent of items above and below the staves. Performing the kind of collision avoidance, including tucking things close together and ensuring that the program doesn’t use more space than it really needs, that Dorico does on a staff-by-staff and system-by-system basis in page view is computationally expensive. The key to Dorico’s fast performance is that we carefully work out the smallest possible range to recalculate after each edit, which typically results in at least the current system being reprocessed at least partially (there are many phases to Dorico’s processing, and this is a simplification, but hopefully you get the basic idea).
Galley view is, to Dorico, effectively a single system. So if we were to run the same processing we do in page view after each edit performed in galley view, the range would normally be widened out to the whole system, which of course equals the whole flow. This means that as the flow gets longer, the amount of computation required to calculate the collision avoidance grows, and thus the program would get slower and slower. It’s not practical to perform the calculation for only part of the flow, because then you would suddenly find that the staves don’t join up at some point in the music where the calculations produce different results.
What we will hopefully be able to do in future is allow you to adjust the spacing between staves in galley view manually, most likely by dragging the little snippet of the staff that appears in the margin to show you the current clef and key signature for each instrument.
In the meantime, if you really want to work in a galley view-like fashion but want collision avoidance, try making a new layout that contains only the flow(s) you’re working on, and define a custom page size that is wide enough to fit more bars into a system than you can normally fit on a regular page. Depending on the complexity of the music and the length of the flow, and the performance of your own system, you may well be able to make a page that’s wide enough to feel like galley view for much of the time with the benefits of collision avoidance from page view but without slowing the performance down too much. Your mileage may vary, however, and this isn’t really something I would recommend. Certainly if you do this and make a page wide enough to fit 200 bars and then complain about slow performance, you won’t get a huge amount of sympathy from me. (And you won’t get the useful margin when the clef etc. is out of view either.)