This issue happened somewhat regularly with Rhino v5, and it seems to be happening with v6 as well.
I have Rhino opened up into a full screen on one of my monitors. I have Grasshopper maximized on the other monitor.
If I am in Grasshopper working away, and then click back into the Rhino viewport to do some Rhino command, I’m usually able to type the command in Rhino using the keyboard and the keystrokes show up in the command prompt window. However, I lose the ability of the Return key to work. Basically, I can type the command but cannot execute it. Similarly, I may lose the function of the Delete key on the keyboard, or perhaps the Esc key. I can go back into Grasshopper and keep working but the keyboard still won’t work if I flip back into Rhino.
Occasionally, I may be able to save and close Grasshopper, then go into Rhino (the only window left on either screen) and the keyboard may work. At other times, even closing Grasshopper will not give me back full control of the keyboard in Rhino.
What’s going on?