The way RadSplitter is applying the borders to its elements is quite complex and the size of the borders is used when the control is calculating its size. For example, some of the panes have applied only right and bottom border to their element while the top and left border is covered from the splitter's element.
Since you have customized the appearance of the borders, could you please provide the exact customizations that you have applied so we can examine them?
the Telerik team
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