Code view can show event handlers for deleted controls

After editing a control's event handler, switching to Designer, deleting the control and returning to Code, Studio can still display the deleted control's handler and its code.

Deleting a control should remove its event handlers from the project and clear any editor state that points to those deleted handlers, or select a surviving member. Returning to Code should never display an editable handler for a control that no longer exists.

Undo should restore the control and its handler code. Redo should remove them again without leaving the deleted handler visible.