[pitivi] timeline: Clarify why the Timeline is an EventBox

[Alexandru Băluț <alexbalut src gnome org>]

commit 5bd8948ca0366e6a52d6b652bee067d629999df1
Author: Alexandru Băluț <alexandru balut gmail com>
Date:   Mon Oct 31 13:24:11 2016 +0100

    timeline: Clarify why the Timeline is an EventBox
    
    Reviewed-by: Thibault Saunier <tsaunier gnome org>
    Differential Revision: https://phabricator.freedesktop.org/D1431

 pitivi/timeline/timeline.py |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)
---
diff --git a/pitivi/timeline/timeline.py b/pitivi/timeline/timeline.py
index b4543b4..dea139f 100644
--- a/pitivi/timeline/timeline.py
+++ b/pitivi/timeline/timeline.py
@@ -315,7 +315,11 @@ class Timeline(Gtk.EventBox, Zoomable, Loggable):
         self.props.expand = True
         self.get_accessible().set_name("timeline canvas")
 
-        # A lot of operations go through these callbacks.
+        # A window is needed to receive BUTTON_* events. This is the reason why
+        # Timeline is a Gtk.EventBox subclass and not directly a Gtk.Box,
+        # see `hbox` above.
+        assert self.get_has_window()
+        # A lot of operations go through the handlers of these events.
         self.add_events(Gdk.EventType.BUTTON_PRESS | Gdk.EventType.BUTTON_RELEASE)
 
         # Whether the entire timeline content is in view and