If you don't want a system-wide change, you can set the following setting. It should affect only applications running on JDK, run this in terminal:
defaults write net.java.openjdk.cmd "AppleWindowTabbingMode" never
It turns out that is a problem related to the AppleWindowTabbingMode
settings. I was using "always", since I want that apps like Finder and
other use new tabs rather than new windows.
This settings worked fine with NetBeans since Catalina. So I reset the
behavior to "never" only for NetBeans, with the follow command
defaults write net.java.openjdk.cmd "AppleWindowTabbingMode" never
With this setting NetBeans works finally fine!
Sources: apparently, it's a bug, please see here and corresponding JDK bug.
Also, it's not a problem with NetBeans only. It affects AndroidStudio also see here for example: