if HdyLeaflet is not the go-to way to implement a convergent side panel, then what is?
I'm not sure if HdyLeaflet is supposed to be an equivalent of GtkPaned, and if not, I hope that HdyPaned is planned.
However, if it is supposed to cover GtkPaned usecases, then it probably shouldn't pretend that the only usecase is a list of handles that serve to access different pages. I don't believe that the only use of a side panel equivalent ought to be to select which page displays in the main view.
There are many more usecases for a side panel, and I've always thought that GTK tried to empower the developers, not to force usecases onto them.
Possible solution: