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// Temp variable to store a previously focused control
private Control focused = null;

private void splitContainer_MouseDown(object sender, MouseEventArgs e)
{
   // Get the focused control before the splitter is focused
   focused = getFocused(this.Controls);
}

private Control getFocused(Control.ControlCollection controls)
{
   foreach (Control c in controls)
   {
      if (c.Focused)
      {
         // Return the focused control
         return c;
      }
      else if (c.ContainsFocus)
      {
         // If the focus is contained inside a control's children
         // return the child
         return getFocused(c.Controls);
      }
   }
   // No control on the form has focus
   return null;
}

private void splitContainer_MouseUp(object sender, MouseEventArgs e)
{
   // If a previous control had focus
   if (focused != null)
   {
      // Return focus and clear the temp variable for 
      // garbage collection
      focused.Focus();
      focused = null;
   }
}