This behavior occurs if the RadPane's / RadSplitter's parent is initially 'hidden' (i.e. have set CSS property display:none;). In such scenario RadSplitter cannot calculate properly its size and to avoid it you need to manually execute RadSplitter's repaint() client-method when the parent is made visible.
If this is not the case, please make sure that all of the RadSplitter's parents have set exclusively height property (including <html>, <body> and <form>). You can find more detailed information on the subject in this help article
If you still experiencing problems implementing one of above mentioned approaches could you please open a support ticket and provide a sample runnable project that demonstrates the problem so we can investigate it further?
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