TrayViewport: handle the case of the widget not yet allocated in the size-allocate callback - SL #4279

In the callback, the allocation can be 1x1 if it is yet unknown [1].
This is the case of trays that start hidden by default, like the one
in Browse activity.

[1] http://developer.gnome.org/gtk3/stable/GtkWidget.html#gtk-widget-size-allocate

Signed-off-by: Manuel Kaufmann <humitos@gmail.com>
Signed-off-by: Manuel Quiñones <manuq@laptop.org>
Acked-by: Simon Schampijer <simon@laptop.org>

src/sugar3/graphics/tray.py

@@ -137,6 +137,12 @@ class _TrayViewport(Gtk.Viewport):
             return self._can_scroll_prev
 
     def _size_allocate_cb(self, viewport, allocation):
+        if allocation.width == 1 and allocation.height == 1:
+            # HACK: the first time this callback is called 'width' and
+            # 'height' are 1 so we mark the Viewport as scrollable and
+            # we show the Prev / Next buttons
+            return
+
         bar_minimum, bar_natural = self.traybar.get_preferred_size()
 
         if self.orientation == Gtk.Orientation.HORIZONTAL: