anchor.diff:              addTreeViewNode form entity world
   
       let private populateGroupTabs (form : GaiaForm) world =
  +
  +        // add groups imperatively to preserve existing group tabs 
  +        let groups = World.getGroups Simulants.EditorScreen world
           let groupTabPages = form.groupTabs.TabPages
  -        groupTabPages.Clear ()
  -        let groups = World.getGroups Simulants.EditorScreen world
           for group in groups do
               let groupNameStr = Name.getNameStr group.GroupName
  -            groupTabPages.Add (groupNameStr, groupNameStr)
  +            if not ^ groupTabPages.ContainsKey groupNameStr then
  +                groupTabPages.Add (groupNameStr, groupNameStr)
  +    
  +        // remove groups imperatively to preserve existing group tabs 
  +        for groupTabPage in groupTabPages do
  +            if Seq.notExists (fun (group : Group) -> Name.getNameStr group.GroupName = groupTabPage.Name) groups then
  +                groupTabPages.RemoveByKey groupTabPage.Name
  +
   
       let private setTreeViewSelectionToPropertyGridSelection (form : GaiaForm) =
           match form.propertyGrid.SelectedObject with
  
positive.code:             if World.containsEntity entityTds.DescribedEntity world
              then form.propertyGrid.Refresh ()
              else deselectEntity form world
          | _ -> deselectEntity form world
  
      let private refreshTreeView (form : GaiaForm) world =
          let treeState = getExpansionState form.treeView
  
negative.code:         form.quickSizeToolStripButton.Click.Add (handleFormQuickSize form)
          form.resetCameraButton.Click.Add (handleFormResetCamera form)
          form.reloadAssetsButton.Click.Add (handleFormReloadAssets form)
          form.groupTabs.Deselected.Add (handleFormGroupTabDeselected form)
          form.groupTabs.Selected.Add (handleFormGroupTabSelected form)
          form.rolloutTabControl.SelectedIndexChanged.Add (handleRolloutTabSelectedIndexChanged form)
          form.createEntityComboBox.SelectedIndexChanged.Add (handleCreateEntityComboBoxSelectedIndexChanged form)
  