Flickering cursor with Gtk

#!/usr/bin/env python
"Grab the window cursor and set it to be a pixmap. This flickers; why?"
from gi.repository import Gtk, Gdk, GdkPixbuf, GLib
import cairo

def grab(button, window):
    display = window.get_screen().get_display()
    pointer = display.get_device_manager().get_client_pointer()

    # Create a simple filled 100x100 rectangle as the cursor pixbuf
    pb = GdkPixbuf.Pixbuf.new(GdkPixbuf.Colorspace.RGB, False, 8, 100, 100)
    surface = Gdk.cairo_surface_create_from_pixbuf(pb, 0, None)
    context = cairo.Context(surface)
    context.set_source_rgba(1, 0.5, 0.5, 1)
    context.rectangle(0, 0, 100, 100)
    context.fill()
    pbdrawn = Gdk.pixbuf_get_from_surface(surface, 0, 0, surface.get_width(), surface.get_height())

    # Create the cursor, and grab the mouse and set that cursor
    cursor = Gdk.Cursor.new_from_pixbuf(display, pbdrawn, 100, 100)
    pointer.grab(
        window.get_window(),
        Gdk.GrabOwnership.NONE,
        True,
        Gdk.EventMask.BUTTON_PRESS_MASK | Gdk.EventMask.POINTER_MOTION_MASK | Gdk.EventMask.SCROLL_MASK,
        cursor,
        Gdk.CURRENT_TIME)

    # After five seconds, quit
    GLib.timeout_add(5000, ungrab, pointer)

def ungrab(pointer):
    pointer.ungrab(Gdk.CURRENT_TIME)

w = Gtk.Window()
btn = Gtk.Button("Change cursor")
btn.connect("clicked", grab, w)
b = Gtk.Box()
b.pack_start(btn, True, True, 50)
w.add(b)
w.show_all()
w.connect("destroy", Gtk.main_quit)
Gtk.main()