"flowing" screen in Galley view possible?

Dear Dorico community!

A strange thing happend last time! In a way I was able to fix the playhead in Galley view and the whole score was moving (“flowing screen”) (Dorico Pro 5.1.21)

But later I wasn’t able to find out how I was able to do this…!! This feature I urgently should have…

Who could help? Is there a key command to do this again?

Thank you very much!!!