It solves this problem:
https://forum.cockos.com/showthread.php?t=215017
Also mentioned here:
https://forum.cockos.com/showthread.php?t=190576
And further up in this thread.
Default behaviour:
When you click on an FX in the mixer that's already open (but hidden behind the mixer or other plugins), the plugin is closed.
My suggested solution:
If plugin GUI is closed, open it.
If plugin GUI is open, bring it to the front.