Hello,

I'm seeing a lot of these GTK+ warnings which come from what I believe
might be a bug in GtkBox:

   Gtk-WARNING **: gtk_widget_size_allocate(): attempt to allocate widget with width -700975964 and height 400

I debugged the code a bit and I think the problem is here (I turned
this into pseudocode for simplicity):

         ------------------------------------------------
gtk_box_size_allocate_no_center (GtkWidget           *widget,
                                 const GtkAllocation *allocation)
{
  GtkRequestedSize *sizes;
  sizes = g_newa (GtkRequestedSize, nvis_children);

  /* Retrieve desired size for visible children. */
  for (i = 0, children = private->children; children; children = children->next)
    {
      if (!_gtk_widget_get_visible (child->widget))
	continue;

      sizes[i] = gtk_widget_get_preferred_width_for_height(...);
    }

  /* Allocate child sizes. */
  for (i = 0, children = private->children; children; children = children->next)
    {
      if (!_gtk_widget_get_visible (child->widget))
	continue;

       child_allocation = sizes[i];

       gtk_widget_size_allocate_with_baseline (child->widget, &child_allocation, baseline);
    }
}
         ------------------------------------------------

So if a child is not visible we don't get its desired size (i.e. we
leave sizes[i] uninitialized) but later we don't allocate its size
either so that's not a problem.

However allocating the size of a visible child emits "size-allocate",
and its signal handler may end up showing a sibling that was
previously hidden, causing its child_allocation to be based on an
uninitialized sizes[i].

I found this with a GtkScrollbar that shows or hides itself after a
GtkAdjustment::changed signal from a sibling widget inside the same
container.