Somebody on the #gtk IRC channel asked if it was possible to make a Gtk window transparent. The general consensus was that is was no possible, which of course compelled me to prove otherwise. Here is the solution, which turned out not to be very complicated, but took some time to figure out.

Not sure what this could be used for. Maybe a little tool for selecting a region for taking a screen capture?

Result

Code

transparent_window.py
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#!/usr/bin/env python

import cairo
import gi

gi.require_version('Gtk', '3.0')
gi.require_version('Gdk', '3.0')

from gi.repository import Gtk
from gi.repository import Gdk

class TransparentWindow(Gtk.Window):
    def __init__(self):
        Gtk.Window.__init__(self)

        self.set_size_request(300, 220)

        self.connect('destroy', Gtk.main_quit)
        self.connect('draw', self.draw)

        screen = self.get_screen()
        visual = screen.get_rgba_visual()
        if visual and screen.is_composited():
            self.set_visual(visual)

        self.set_app_paintable(True)
        self.show_all()

    def draw(self, widget, context):
        context.set_source_rgba(0, 0, 0, 0)
        context.set_operator(cairo.OPERATOR_SOURCE)
        context.paint()
        context.set_operator(cairo.OPERATOR_OVER)

TransparentWindow()
Gtk.main()

The example file can be downloaded from the GitHub Gist located here.