To be honest, I'm unsure if this is a TreeView or Splitter problem, so please bare with me.
I have a TreeView in a RadPane. The TreeView, when expanded, puts some of the nodes close to, or past the RadPane's edge. If the overlapping nodes are neither selected or hovered over, all is OK, but otherwise, well you can see for yourself in ss3.gif (attached).
FYI, this happens in IE8 but not FF3.5. The skins are Q1 2009's WebBlue.
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Stuart