You have not cofigured the RadSplitter correctly to occupy 100% of the Height of its parent because that parent does not have specified height. Thus you break the following rule:
If a web page element has its height defined in percent, its parent element must have a height style as well. The rule applies recursively.
You can read about this rule on the W3 site:
Specifies a percentage height. The percentage is calculated with respect to the height of the generated box's containing block. If the height of the containing block is not specified explicitly (i.e., it depends on content height), and this element is not absolutely positioned, the value computes to 'auto'. A percentage height on the root element is relative to the initial containing block. "
You can refer to this online article for information as well.
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