There are three possible ways in which you can configure the RadSplitbar's resize mode, depending on the way you want the size ot the collapsed pane to be distributed. You can find detailed information about them and the differences in their functionality in the following live demo: Resize Modes
Please note, though, that if you want to keep the width of the both expanded panes the same like before the collapsing - I am afraid that this is not possible. The size of the Splitter has to remain the same even after the collapsing of any of its panes.
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