This issue will keep track of everything needed to fully implement the History API

The History interface:

• ScrollRestoration enum and attribute
• length attribute (Implement history.length #12685)
• go([ delta ]) method (History interface Go, Back, and Forward #12552)
• back() method (Implement History's back/forward APIs #5670 History interface Go, Back, and Forward #12552)
• forward() method (Implement History's back/forward APIs #5670 History interface Go, Back, and Forward #12552)
• state attribute (Implement History State #20638)
• pushState(data, title [, url ]) method (Implement History State #20638)
• replaceState(data, title [, url ]) method (Implement History State #20638)
• traverse the history by a delta (History interface Go, Back, and Forward #12552)
• Make these [Throw] if the document is not fully active. (Make History attributes and methods throw #16697)

Spec: https://html.spec.whatwg.org/multipage/browsers.html#the-history-interface

Task Source and Events

• Fully implement HistoryTraversalTaskSource (Fully implement HistoryTravesalTaskSource #10994)
• PopStateEvent (Implement navigation events #11005 Implement navigation events #11006)
• HashChangeEvent (Implement navigation events #11005 Implement navigation events #11006)
• PageTransitionEvent (Implement navigation events #11005 Implement navigation events #11006)

Specs:
https://html.spec.whatwg.org/multipage/webappapis.html#history-traversal-task-source
https://html.spec.whatwg.org/multipage/browsers.html#the-popstateevent-interface
https://html.spec.whatwg.org/multipage/browsers.html#hashchangeevent
https://html.spec.whatwg.org/multipage/browsers.html#pagetransitionevent

Browsing Context (Edit: Now WindowProxy)

• Replace Page with BrowsingContext (Replace Page with BrowsingContext #11031 Combine Page into BrowsingContext #11044)

History State

• Create a StateId type to refer to the associated JS state for a session history entry. Serialized history states should be stored in a resource thread and freed when no longer accessible. Once a state becomes activated, it must be deserialized and stored by the History object. (Implement History State #20638)

Constellation

• Setup a joint session history that has all of the session history entries interleaved by the time they were created (https://github.com/ConnorGBrewster/ServoNavigation) (Implement joint session history #11866)
• Add delta to NavigationDirection (Add support for navigating by a delta #11616)
• Allow multiple session history entries to share the same pipeline id.

URLs and Hash Change

• History state diffs should track URL changes
• Scrolling to fragment should trigger a hash change event and add a new diff in the session history
• Hash change events should be fired when the hash changes during a history traversal.

Scroll Restoration

• Save scroll offset in session history and restore the scroll location on navigation depending on the value of ScrollRestoration

Testing

• Add more tests for the History API that test many edge cases not checked by the current suite. (https://github.com/cbrewster/ServoNavigation)