The problem of the 'unmet dependencies' on elementaryOS 6.0 appears to be solved when installing the elementaryOS 6.0 public beta from their April 30 build (I was still on an apt-updated install of the April 28 build before).
So it appears that we can use the 6.0 public beta to work on elementaryOS apps at the moment.
You are adding the stack to the window:
You are then constructing an object called grid that you're packing the StackSidebar into, but then the grid doesn't get added to the window.
So you probably need add (grid) instead.
You seem to be missing one last step, actually calling the function that is supposed to save the new values in the gsettings schema. To do this, you need to connect the delete_event signal to your before_destroy () function. I like to place that right before calling show_all ():
Also, the before_destroy () function must return false for the window to ...