I just tried the game on my second computer, a mac with macOS 10.14.6 and two monitors at 1920x1080. The game starts fine and goes to full screen mode, but if I tab out (Command-tab) to switch to another application, or when the game does this when you press the forum or patch-notes button, the game window disappears and doesn't come back if I try to switch back to it. The game application is activated, but it doesn't go back to fullscreen showing the actual game. If you set the option to only play music when the game is active, the music starts again when you tab back to the game, but nothing is shown.
At this point the menu bar is hidden as well, as if the game was in fullscreen mode. But it still responds to the menu key commands (Command Q to quit for example). If I at this point use command F to toggle fullscreen to windowed mode it seems to toggle to window mode, but no window is shown. However, if I then toggle back to fullscreen with Command F again it goes into fullscreen, finally showing the game again, but at a lower resolution and not the native 1920x1080 I had before.
I'm not sure if this is due to the resolution or the fact that this computer has two monitors, or if this is a bug introduced in a recent patch. I've previously played the game on another mac with the same macOS version, but a single higher resolution monitor, and there it worked fine and I could tab in/out with no problem.
Start the game, tab out from the main menu, tab back in.