This has happened 3 or 4 times. All of a sudden the orange selection stops being visible (i.e. the notes remain black even though they’re definitely still selected). Only solution I’ve found so far is to restart Dorico. As far as I can tell it happens at random times. Attached shows me block selecting a bunch of notes and hitting DELETE. Although they are selected and deleted, they never turn orange.
I believe this might be caused by the “instant print preview” feature getting stuck on, somehow. You might not even be using that feature deliberately; on an English keyboard it uses the key to the left of Z on the bottom letter row. Try pressing that to see if it causes the selection to reappear.
Are you talking about the left SHIFT key??
It’s a shift key on a U.S. keyboards. The reverse-single-quote on my U.S. keyboard is above the TAB key on the same key as the tilde.
Search in the Key Commands page of Preferences for “hide invisibles”, which will show you what key to type on your speciific keyboard.