Merge pull request #2827 from jasongrout/tabselection

Fix the logic for tab selection when the children changes to select a tab when possible

Author: vidartf

ipywidgets/widgets/widget_selectioncontainer.py

@@ -52,6 +52,22 @@ def __init__(self, **kwargs):
             kwargs['selected_index'] = 0
         super(Tab, self).__init__(**kwargs)
 
+    @observe('children')
+    def _observe_children(self, change):
+        # if there are no tabs, then none should be selected
+        if len(change.new) == 0:
+            self.selected_index = None
+
+        # if there are tabs, but none is selected, select the first one
+        elif self.selected_index == None:
+            self.selected_index = 0
+
+        # 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
+
+
 @register
 class Stacked(_SelectionContainer):
     """Displays only the selected child."""