The default behavior for the splitter is to take focus once selected. If you want the splitter to be movable without taking focus you can do one of two things:

1. Use the custom control defined in the SplitContainerNoFocus sample
2. Insert the following code in your project, and attach these events to all of the SplitContainers that you don't want stealing focus.

// 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;
   }
}