Mac shortcut for next window - IDE 1.8.7 problem
In MacOS 10.14 and Arduino IDE 1.6.12, cmd` cycles through 5+ IDE windows properly, for me.

Update:


  When you upgrade to Arduino 1.8.7,


I won't be. I've tried a couple of its point releases and have given up on it because its launch performance is so p...-poor. Maybe 1.9 will finally fix it.

Sloeber/Eclipse is my serious code-crunching IDE. I keep the A-IDE around for reference and for quick knock-off experiments because it's quick to launch.

Update 2:

The OP asked me to note that he 


  found that  Cmd ⌘   Shift   `  cycles
  through all windows (maybe in reverse).


Update 3:


  In older IDE the reverse cycling with Shift doesn't work?


It works in both directions for me (shifted/unshifted).