There is a workaround, however it is not fully tested and there may be side effects. Still in the usual case it works just fine.
Attach to the OnClientResized event of the sliding pane and have the following event handler:
pane.getContentContainer().style.width = pane.getContentContainer().parentNode.offsetWidth +
The idea is to set the width of the pane's content the same as that of the parent, which defines where the splitbar is.
Hope this helps.
the Telerik team
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