I built up a test demo page based on your code but unfortunately to no avail - I could not reproduce the issue on my side. What I can suggest is the following:
1) Make sure that you have the following style in your page's head:
html, body, form
2) Make sure that you are using the latest version of RadControls because there were similar issues with older versions of RadSplitter.
I attached my test page for your reference - let me know if you by any chance reproduce the issue with it with the latest version and if so provide more information about the browser, the exact reproduction steps, etc and I will do my best to help.
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