To reproduce:

Add a RadTreeView (or any other control with tooltips) and enable the tooltips. Set the ToolTipText to be long and move the form to the end of the screen. Hover over an element to show a tooltip, you will see that the Tooltip is cut off.

Workaround:

Move the ToolTip manually:

void TreeView_ToolTipTextNeeded(object sender, ToolTipTextNeededEventArgs e)
{
    e.ToolTipText = "SOMEEEEE LOOOOONNGGGG TOOOOLLTIIIPPP TTTEEEXXXTTT";


    Size screenSize = new Size(Screen.AllScreens[Screen.AllScreens.Length - 1].Bounds.Width * Screen.AllScreens.Length, Screen.PrimaryScreen.Bounds.Height);
    Size textSize = TextRenderer.MeasureText(e.ToolTipText, new Font("Courier New", 10.0f, FontStyle.Bold));
    Point mousePoint = MousePosition;


    Point toBeDisplayedTooltipLocation = new Point(mousePoint.X + textSize.Width, mousePoint.Y);
    if (toBeDisplayedTooltipLocation.X >= screenSize.Width)
    {
        e.Offset = new Size(screenSize.Width - toBeDisplayedTooltipLocation.X, 0);
    }
}