Following the upgrade of IGB to use Java 21, there are additional steps to run IGB through Netbeans with a debugger:

Start IGB through a terminal on Mac/Linux

sh run_igb.sh -d 

Start IGB through a terminal on Windows

sh run_igb.bat -d 

In Netbeans, attach a debugger by going to Debug > Attach Debugger…
In the window that opens, enter the following values:

Debugger: Java Debugger (JPDA)
Connector: SocketAttach (Attaches by socket to other VMs)
Transport: dt_socket
Host: localhost
Port: 5005

Then click OK