Hi new to this forum but thought I would add some info I have found. This is frankly one of the most bizarre and frustrating issues I have ever experienced and I have been making music on DAW’s for a lot of years professionally.
Firstly I never had the problem with Cubase 9 pro on El Capitan for sure. By coincidence when I upgraded to High Sierra 10.13.3 and downloaded Cubase Pro 9.0.30 my old wired keyboard and mouse gave up the ghost (sticky keys etc), so I started using an Apple Magic Mouse and wireless keyboard. Then the problem started. Same as everyone else, couldn’t double click and name tracks, and couldn’t open the midi editor by double clicking. Tried everything, including totally wiping my hard drive and doing a fresh install of everything. Problem wouldn’t go away. I then got my old wired keyboard and mouse combo cleaned and refurbished on the off chance and installed it. Problem went away immediately.
Oddly I know other users I work with who have the exact same Mac as mine (Mac Pro late 2013, 3.7 Ghz quad core, 64 gb ram) running Cubase 9 Pro on High Sierra who don’t get the problem using wireless Apple keyboard and mouse - so go figure that one!
Anyway all I can say, which I hope might help other users is try reverting to an old wired keyboard and mouse (and I hate that old wired Mac mouse but I hate the double clicking issue more!!) to see if it gets you out of trouble.