Extending from GtkBin

I'm trying to make a custom widget that resembles the "quick search" entry that Gtk uses on all TreeView-like widgets.

Here's a simplified example of my initial idea:

from gi.repository import Gtk

class QuickSearch(Gtk.Bin):
    def __init__(self, *args, **kwargs):
        super(QuickSearch, self).__init__(*args, **kwargs)
        self.add(Gtk.Entry())

win = Gtk.Window()
win.connect("delete-event", Gtk.main_quit)
search = QuickSearch()
win.add(search)
win.show_all()
Gtk.main()

The problems are the following:

If I use it alone, as in this example, there seems to be space allocated to the custom widget, but the GtkEntry is not shown on the screen. I tried various combinations of properties on the GtkEntry and the custom widget (hexpand, vexpand, margins, aligns, etc) with no avail.
If combined with other widgets, when I interactively query for the widget allocated height, it aparently is 1. On the screen, the widget effectively doesn't appear (since it's somewhat "squashed" between the other widgets).

So, there's something I'm missing on the object initialization or how I'm using this custom widget? Is this a problem specific to GtkBin? I tried doing the same with a GtkBox and works flawlesly, but I think GtkBin is better suited for this particular widget.

For some reason, Gtk.Bin is not allocating space to its child. If you implement the do_size_allocate virtual method then it works. Why the default implementation is not being called, I have no idea.

from gi.repository import Gtk, Gdk

class QuickSearch(Gtk.Bin):
    def __init__(self, *args, **kwargs):
        super(QuickSearch, self).__init__(*args, **kwargs)
        self.add(Gtk.Entry())

    def do_size_allocate(self, allocation):
        self.set_allocation(allocation)
        child_allocation = Gdk.Rectangle()
        border_width = self.get_border_width()
        child_allocation.x = allocation.x + border_width
        child_allocation.y = allocation.y + border_width
        child_allocation.width = allocation.width - 2 * border_width
        child_allocation.height = allocation.height - 2 * border_width
        self.get_child().size_allocate(child_allocation)

win = Gtk.Window()
win.connect("delete-event", Gtk.main_quit)
search = QuickSearch()
win.add(search)
win.show_all()
Gtk.main()

However, I think inheriting from a Gtk.Frame is more appropriate for your purpose; it is a one-child container with a styleable border around it, which is what you want and more. And its size allocate method works.