Python GTK 3 button-press-event triggering twice

I create a Gtk.Window and connect the "button-press-event". When this window is clicked, the event is triggered twice. Minimal example:

import gi

gi.require_version("Gtk", "3.0")

from gi.repository import Gtk

w = Gtk.Window()
w.set_size_request(800, 800)
w.connect("button-press-event", lambda a, b: print(b))
w.show_all()

Gtk.main()

This prints b twice. At first I thought it was somehow a press/release but, as far as I can tell, the b object is the same both times (it's a different instance, but with the same values). Anyone know what is going on?

I'm using python 3.8.10 and gi version 3.36.0.