listbase: Don't hand out invalid positions

gtk_list_base_get_position_from_allocation relies on
the rbtree allocations being valid, and thus can't really
be used more than once before doing a size_allocate. We
generally queue a resize when we invalidate the rbtree
in response to handling input events, so we can use the
resize_needed flag as a proxy for the tree being invalid.

This change prevents the jumpy scrolling that is currently
happening when we manage to get two scroll events during
a single frame cycle, where the scrollbar sometimes jumps
to the other end of a long list.

gtk/gtklistbase.c

@@ -132,6 +132,9 @@ gtk_list_base_get_position_from_allocation (GtkListBase           *self,
                                             guint                 *pos,
                                             cairo_rectangle_int_t *area)
 {
+  if (widget->priv->resize_needed)
+    return FALSE;
+
   return GTK_LIST_BASE_GET_CLASS (self)->get_position_from_allocation (self, across, along, pos, area);
 }