Emulating a block cursor in GtkSourceView

[Tony Houghton <h realh co uk>]

I'm trying to emulate a block cursor in GtkSourceView by superimposing a
rectangle over it. I registered a handler for expose-event with
g_signal_connect_after, and it does get called with expected
coordinates, but no rectangle appears. I'm trying to draw it with
gdk_draw_rectangle on the widget's window field.

I don't know whether the reason for the failure is because I haven't set
up the GdkGC properly (do I need to set a clip mask pixmap? - I don't
understand how; does leaving it blank draw everything?) or something or
if you just can't draw on top of widgets this way.

Can anyone help? Is this a generic widget issue so I'd be better off
asking in the gtk-app-devel list? The essentials of my code (in python)
are below.

    @staticmethod
    def eor_colours(c1, c2):
        return gtk.gdk.Color(c1.red ^ c2.red, \
                c1.green ^ c2.green, c1.blue ^ c2.blue)

    ...

    def set_style(self, widget = None, old_style = None):
        ...
        # self.view is the gtksourceview2.View
        self.win = self.view.get_window(gtk.TEXT_WINDOW_WIDGET)
        ...
        c = self.eor_colours(style.base[gtk.STATE_NORMAL], self.colour)
        self.norm_gc = gtk.gdk.GC(self.win, c, c, None,
                gtk.gdk.XOR, gtk.gdk.SOLID,
                line_width = 1, line_style = gtk.gdk.LINE_SOLID)

    ...

    def enable(self):
        ...
        if self.expose_tag == None:
            self.expose_tag = self.view.connect_after("expose-event",
                    self.expose_handler)
        ...

    # ... there's some code to work out the cursor position and store it
    # in self.rect and call self.win.invalidate_rect() whenever it
    # moves, blinks or otherwise needs updating

    def expose_handler(self, widget, event):
        if self.blinked_off or not self.rect or not self.enabled:
            return True
        intersect = event.area.intersect(self.rect)
        if intersect.width and intersect.height:
            if not self.norm_gc:
                self.set_style()
            if self.in_selection:
                gc = self.sel_gc
            else:
                gc = self.norm_gc
            self.win.draw_rectangle(gc, self.has_focus,
                    self.rect.x, self.rect.y, self.rect.width, self.rect.height)
        return True     # I've tried False here, no apparent difference

-- 
TH * http://www.realh.co.uk