At first glance the set_resize_start_child and set_resize_end_child functions seem useful, however, can’t you just use expand and alignment properties in the children to control this instead?
I tried to get the pane separator to stick to a side and resize the widgets inside, and the only way I could do it was by setting the resize to False then having the child use Gtk.Align.FILL . Out of curiosity, I checked what else I could do with resize that I couldn’t do with align and extend, and I didn’t find anything
The point of “resize” is to control which side will grow/shrink when the total allocation (typically the application window) changes.
Let’s suppose the GtkPaned is directly inside a GtkWindow, the start child has resize=False and the end child has resize=True. If you resize the window, you’ll see the start child keeping its size.
If on the other hand you only use the “align” properties, the start widget will probably just stay at its minimum size, no more no less.