What is a ConcertinaPanel?

Can sombody explain what a ConcertinaPanel exactly is? What is its behaviour?

It’s the thing on the left of the projucer, I believe.

If you have VSCode handy, a good technique is to do a ‘find in folder’ on the JUCE folder and search for the class name. 90% of the time, you will find a code example in there somewhere.