I am afraid that the provided screenshot is not enough to completely understand the problem and also as far as we see you have some additional customizations which might also cause the problem.
What we can assume is that the element which you have marked is one of the following:
1) Resize bar of the RadSlidingPane
2) The RadSplitBar between the panes
This being said, please disable the resizing of the sliding pane and remove the splitbar and test again whether the additional pixels disappear.
If this does not help, please prepare very simple sample reproduction code which is fully runnable and share it here and we will do our best to help.
the Telerik team
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