Using the code Anthony kindly provided here with a few tweaks:
Public Function MouseX(extends r as DesktopUIControl) As Integer
// For the control referenced, figure out the local MouseX position by walking up the containment hierarchy.
Dim retValue As Integer = r.Window.MouseX - r.Left
Dim p As Object = r.Parent
Dim container As DesktopContainer
While p <> r.Window
if p isa DesktopUIControl then
p = DesktopUIControl( p ).Parent
ElseIf p IsA DesktopContainer Then
container = DesktopContainer( p )
retValue = retValue - container.Left
p = container.Parent
end if
Wend
Return retValue
End Function