Asking here because I found no obvious way within Dorico and nothing in the forums:
I have one page that needs to be a bit more dense than the others. I went to Engrave Mode, moved some systems closer together, and now there is enough space at the bottom of the page to hold one more system. How do I get the first system of the next page to move over to the current page?
Set system breaks at the start of each (existing) system on the page (apart from the first one, which already has a frame break).
Set a frame break at the beginning of the second system of the next page - this is the place where you want the next page to start.
Grab the first frame break you inserted, in Step 1, and from the bottom properties panel tell it to wait for next Frame Break.
You might sometimes achieve identical results by just selecting the start and end notes and hitting the “Make into Frame” button - it’s what it’s there for - but my method prevents Dorico from taking completely reflowing the music within the page, which it has a tendency to do.