How to make ZAP Proxy go to background?

I've a problem with ZAP Proxy that prevents it from letting other windows be displayed on top of it. That is every window gets displayed behind the ZAP Proxy main windows; in order to view other windows, the ZAP Proxy main windows must be resized.

The problem is present on macOS Mojave (haven't tested it on other platforms).

How can I fix this?