In the old GtkTreeView, tree items are collapsed/expanded with with Shift + left/right arrow. This is both inconvenient and inconsistent (with other platforms, as well as among GTK apps, some of which override this behavior). There is an open issue https://gitlab.gnome.org/GNOME/gtk/-/issues/717, but it seems that there are no interest in breaking GTK 3.
Would it be possible to change, though, in GTK 4, to using left/right arrow for expanding/collapsing by default? Especially since now people are porting tree views to the new list view. Seems like a good opportunity to introduce this change.
Additionally, pressing left arrow should move the focus to the parent item if the current item cannot be or is already collapsed. And right arrow should move to the first child item if the current item is already expanded.