VERY quick consecutive navigations between pages (Alt+Left Arrow held down for a while to navigate all the history backwards, for example) result in an empty DOM tree. This is caused by the DOMAgent.getDocument() callback not being invoked and WebInspector.DOMAgent's _pendingDocumentRequestCallbacks not being cleared between navigations, which results in callbacks being accumulated there without actually requesting the document from the backend.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.