When scrolling on pitchfork.com, the fullscreen progress is continuously reset to 1.0 (i.e. fully visible toolbar).  This is occurring because the page is continuously performing window.location.replaceState() calls to update the URL.  This is going through FullscreenWebStateObserver::DidFinishNavigation(), which resets the progress to 1.0 to show the toolbar.

It's unclear what the correct fix for this would be.  The NavigationContext passed to the WSO callback contains whether the navigation is same-document, but I think we'd still want to show the toolbar when the document changes for pushState() navigations, so we wouldn't be able to use that as a signal for whether to reset the model.  The web// layer tracks whether a navigation is due to a replaceState() call, but this information is currently private, and would need to be exposed in the public interface in order to differentiate these navigations.  This would be a departure from content//'s public interface.