Still using VB10 on Win7 here, When I have RTB resize with form (anchored on all sides) invalidating the RTB in forms resize-end event works here,...
Code:
Private Sub FrmMain_ResizeEnd(sender As Object, e As EventArgs) Handles Me.ResizeEnd
    RTB1.Invalidate()
End Sub
Other workarounds, might be to toogle the rtb ScrollBars property,
http://www.vbforums.com/showthread.php?793671