So sorry. 
I’ve so stupidly forgotten the name of this vertical line and the shortcut keys that make it appear/disappear - (it’s some kind of bookmark or marker and stops certain changes propagating to the right of it).
Ideally. A hover over a rare object could tell us its name. Once you forget its name you can’t search for it 
- I’ve even looked through all the keyboard map and I still can’t find it.
It’s the stop line, and the toggle command is Shift+Alt+I (Shift+Opt+I on Mac)
Insert Mode Stop Position is the proper name, sorry!
It allows you to lock the music beyond it when you’re in insert Mode, which can be extremely useful.
You can also toggle it on and off from the system track by clicking the 
icon.
thank you so much. I feel so silly forgetting both its name and its shortcut. (My only excuse is that I hadn’t worked on an ‘extended’ work for such a long time and hadn’t needed it).
No one can remember everything in Dorico! And as you can see, I’d forgotten the exact name too 
I found it again by searching the corresponding section in the manual to share it here.
