I can’t find any mention of this after 2017, but I’ve now experienced this in two sessions - the mouse just stops highlighting selections (single and multiple), but the selections are evidently still made, given that I can carry on working without the highlighting. It seems from an old thread that changing mode and back again solves this, but it hasn’t worked for me (though exiting and reopening the project does).
I think in some cases, the Instant Print Preview/Hide Invisibles key can get “stuck”, e.g.
I use the backslash all the time - it’s one of my top Dorico features - though I haven’t intentionally used it for a few sessions now (it comes into its own during the Engrave step). The key itself doesn’t appear to have the propensity to stick, but I’ll check it if this behaviour happens again. It has to be said, though, that the issue was solved by a reopening of the project, and I’m sure I hadn’t touched the backslash key in the meantime.
You should find that simply pressing and releasing the ` (macOS) or ** (Windows) key is sufficient to bring it back to life, without needing to resort to closing and reopening the project.
Oh - I’ll try that if it happens again! Thanks.