This is a bug/issue I've had for quite some time, but have been just enduring until now.
Essentially, I'll be playing in full screen when I decide to change a song or access Google to look something up or vote for a server. When I go out of fullscreen, my window will sometimes disappear. Sometimes the window will appear again without the red, yellow, and green circle buttons on the top left (as is the way of a Mac) and when I try to fullscreen it, it won't work. When this happens, it forces me to click Minecraft from the dock and quit it. It's frustrating to do this so much.
I'm sorry. I'm not sure what information I need to include but I'll do some I think may be helpful.
MacBook Pro (Retina, 13-inch, Mid 2014)
Processor: 3 GHz Intel Core i7
Memory: 16 GB 1600 MHz DDR3
Graphics: Intel Iris 1536 MB
Profile: 1.8.9.forge1.8.9-188.8.131.528-1.8.9 (I use Forge with Optifine, AutoGG, Autotip, WorldEdit, and 5zig)