Let me shed some light on the matter.
The disabled attribute is relevant only for input elements, while the RadButtons wrapper is an anchor element. This means that, according to specifications, setting disabled="disabled" should not affect its behavior. This works like it is supposed to only in IE and this is why you are experiencing the described behavior.
The RadWindow's OpenerElementID property is a simple one that only attaches an event handler for the onclick event to the target. When the target is disabled this event is not fired by the browser (e.g. for a regular button type submit). For an anchor, however, the event will still be fired and the RadWindow will be shown which is the case with the RadButton.
What I can advise is that you use the OnClientClicked or OnClientClicking events of the RadButton (often along with the AutoPostBack property) to attach a function that will open the RadWindow. This help article
explains how a RadWindow can be opened and this blog post
explains how to work with the RadButton events.
the Telerik team
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