Yes, you were right, thank you.

The solution was to add a connector and to draw the line after showing the window:
...
builder->get_widget("mainwindow")->show_all();

win = builder->get_widget("drawingarea");      // A DrawingArea from a glade file
gc = GTK2.GdkGC(win);
win->set_background(GTK2.GdkColor(255, 0, 0));      // It's ok, the background will be red
gc->set_foreground(GTK2.GdkColor(0, 0, 255));
win->signal_connect("expose-event", lambda() {
       win->draw_line(gc, 1, 1, 100, 100);                // It draws a blue line :) :) :)
});
...

Cheers:
Thomas

> From: 10353@lyskom.lysator.liu.se
> Subject: GTK2 DrawingArea question
> To: pike-devel@lists.lysator.liu.se
> Date: Sun, 2 May 2010 16:15:03 +0000
>
> You probably need to draw the line _after_ the window is shown.
> Windows in X normally don't have backingstore, so you need to redraw
> their contents whenever a previously invisible region becomes
> visible. I imagine there is some signal you should connect to your
> draw function to have it called whenever needed.

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