The select a presentation display algorithm rejects the Promise "if there is already an unsettled Promise from a previous call to start on any PresentationRequest in the same controlling browsing context".

This is done to allow only one user prompt to select a presentation display at a time for a given tab. However, since start can be called by nested browsing contexts as well, I think "the same controlling browsing context" does not capture the notion of "tab" that we need.

There does not seem to be a term to say "all the browsing contexts that compose the tab" in HTML:
http://www.w3.org/TR/html51/browsers.html#nested-browsing-contexts

We could perhaps say "Let top be the top-level browsing context of the current browsing context. If there is already an unsettled Promise from a previous call to start on any PresentationRequest in top or in the list of the descendant browsing contexts of top..."

Issue initially raised in: #392 (comment)