Hello guillaume.lebel,
Please consider removing the AutoPostBack = true declaration from RadListBox should posting back to the server is not desired. The set_cancel property in the SelectedIndexChanging event args serves for canceling the selection of the given item, not for preventing a postback. However, it indeed makes sense that canceling the selectionIndexChanging should not postback the page. We will research what side effects might such change have to RadListBox.
Sincerely yours,
Genady Sergeev
the Telerik team
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