I haven’t seen anyone else post about this but I’ve been experiencing this problem sporadically in Rhino 6. At some point I’ll lose the ability to copy with LMB+Drag+Alt and also to delete with the Delete key. In order to copy or delete I have to go to the menu and do it from there. Restarting Rhino solves the problem - until the next occurrence.
maybe some soft has the same hotkey
Interesting thought but Rhino is the only thing running and I only have problems with the GH canvas.
I`ve had the same problem (in rhino 5 ) had something to do with GH going full screen and floating rhino viewport
When I experience this problem I do notice that when trying to delete or copy, I end up in the Rhino command line, but it doesn’t seem to involve GH going full-screen.
Not a super solution, but holding down the Windows key and pressing D (i.e. switch to desktop) a couple of times usually shakes things into place for me, whenever Rhino/Grasshopper loses its focus like this…
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Thanks Anders. Now I can’t wait to lose focus again so I can test your solution!
I provide no guarantees 
Sorry to say Anders, I encountered the situation again and your suggestion did not work. I wish I knew what was going on because it seems to be happening with greater frequency.
Ah dang, I’m sorry. Hope a solution pops up soon.