Fix selection container logic around setting titles.

Previously, the title logic was being overridden by Tab. This refactors the code so we can just override the selected index logic, without overriding the titles logic.

Fixes #3520

Author: jasongrout

python/ipywidgets/ipywidgets/widgets/widget_selectioncontainer.py

@@ -41,9 +41,15 @@ def _validate_titles(self, proposal):
 
     @observe('children')
     def _observe_children(self, change):
-        if self.selected_index is not None and len(change.new) < self.selected_index:
+        self._reset_selected_index()
+        self._reset_titles()
+
+    def _reset_selected_index(self):
+        if self.selected_index is not None and len(self.children) < self.selected_index:
             self.selected_index = None
-        if len(self.titles) != len(change.new):
+
+    def _reset_titles(self):
+        if len(self.titles) != len(self.children):
             # Run validation function
             self.titles = tuple(self.titles)
 
@@ -90,10 +96,10 @@ def __init__(self, **kwargs):
             kwargs['selected_index'] = 0
         super(Tab, self).__init__(**kwargs)
 
-    @observe('children')
-    def _observe_children(self, change):
+    def _reset_selected_index(self):
         # if there are no tabs, then none should be selected
-        if len(change.new) == 0:
+        num_children = len(self.children)
+        if num_children == 0:
             self.selected_index = None
 
         # if there are tabs, but none is selected, select the first one
@@ -102,8 +108,9 @@ def _observe_children(self, change):
 
         # if there are tabs and a selection, but the selection is no longer
         # valid, select the last tab.
-        elif len(change.new) < self.selected_index:
-            self.selected_index = len(change.new) - 1
+        elif num_children < self.selected_index:
+            self.selected_index = num_children - 1
+
 
 
 @register