WPT details: Since the feature depends on whether the UA performs a visual transition on navigations (as a part of browser UX), it's difficult to write an interoperable WPT for this. The exact gesture which causes a UA transition is a browser's internal detail.
Smooth visual transitions as users navigate on the web can lower cognitive load by helping users stay in context. However, the user experience is bad if both the site author and the UA add these transitions: the transitions may conflict and cause confusion for the user. This API avoids such cases to ensure only one visual transition is executed at a time. The API adds a boolean on `PopStateEvent` and `NavigateEvent` to indicate whether the UA has executed a visual transition for this navigation. Authors can use this to skip their custom transition.
Smooth visual transitions as users navigate on the web can lower cognitive load by helping users stay in context. It can also provide a visual cue about the destination before initiating the navigation. Both site authors and user-agents (UAs) add visual transitions to their navigations for these use-cases. However, the user experience is bad if both the site author and the UA add these transitions: the transitions may conflict and cause confusion for the user. The goal of this proposal is to avoid such cases to ensure only one visual transition is executed at a time.
Explainers: https://github.com/WICG/view-transitions/blob/main/default-ua-transitions.md