Paint on Transparent window with HeaderBar Python and Gtk3

Question

I have a little problem with the following code. It creates a GtkWindow, make it paintable so I can draw on it with cairo on draw events. Then I add a GtkHeaderBar and a simple button widget.

from gi.repository import Gtk
import cairo

def draw_callback(widget,cr):
  if widget.transparency:
    cr.set_source_rgba(0,0,0,0.5)
  else:
    cr.set_source_rgb(0,0,0)   

  cr.set_operator(cairo.OPERATOR_SOURCE)
  cr.paint()
  cr.set_operator(cairo.OPERATOR_OVER)

win= Gtk.Window()
win.connect('delete-event', Gtk.main_quit)
win.set_app_paintable(True)
screen = win.get_screen()
visual = screen.get_rgba_visual()
win.transparency = False

if visual and screen.is_composited():
  win.set_visual(visual)
  win.transparency = True
else:
  print('System doesn\'t support transparency')
  win.set_visual(screen.get_system_visual)

win.connect('draw', draw_callback)

win.add(Gtk.Button(label='test'))
bar = Gtk.HeaderBar(title='title')
bar.set_has_subtitle(False)
bar.set_show_close_button(True)
win.set_titlebar(bar)
win.show_all()

Gtk.main()

The draw_callback paint the window background but this background looks like this :

It is like the cairo context doesn't have the same size of the window. The part that is draw and that is outside the window doesn't seems to answer to mouse event ( I cannot grab the window from this part for example)

If I don't use an HeaderBar, I don't have this problem.

This works with ruby 2.2 and the Gtk3 bindings.

My python Version is python 3 and the up to date Gtk modules

Edit:

Problem still exist with :

def size_allocation_cb(widget, rectangle):
  widget.x = rectangle.x
  widget.y = rectangle.y
  widget.width = rectangle.width
  widget.height = rectangle.height

win.connect('size-allocate', size_allocation_cb)

def draw_callback(widget,cr):
  if widget.transparency:
    cr.set_source_rgba(0,0,0,0.5)
  else:
    cr.set_source_rgb(0,0,0)   

  cr.set_operator(cairo.OPERATOR_SOURCE)
  cr.rectangle(widget.x, widget.y, widget.width, widget.height)
  cr.fill()
  cr.set_operator(cairo.OPERATOR_OVER)

Answer 1

When you have a header bar, your window is drawn entirely on the client side, including shadows (for some reason). You will need to call cairo_rectangle() or some other function, preferably with the size allocation (not size request!) of the child of the GtkWindow, to clip cairo_paint() to the correct size.