Polyfill that mimics Chrome's scroll restoration behavior

In single page apps that use the history API for navigation, the DOM is typically not completely ready immediately when the popstate event is fired, as opposed to traditional applications where after page load the page is completely filled with content.

This means that the scroll position cannot be set until the AJAX requests have finished loading, and the page is fully rendered. It seems that Chrome implements this behavior by looking at AJAX requests made by the page, and not restoring the scroll position until these have finished, unless it is already possible earlier (e.g. if the page already has the correct height).

See Chrome's implementation.

This polyfill tries to mimic that behavior, though it is a rather limited implementation because of browser differences and insufficient browser APIs.

Check out Brigade.com for an example:

Usage

npm

npm install delayed-scroll-restoration-polyfill --save

In your HTML

<script src="node_modules/delayed-scroll-restoration-polyfill/index.js"></script>

You can also grab the polyfill directly from unpkg.com:

<script src="https://unpkg.com/delayed-scroll-restoration-polyfill@0.1.1/index.js"></script>

How it works

We overwrite history.pushState to record the current scroll position when navigating to a new page.
We also overwrite history.replaceState to avoid overwriting this scroll position.
Finally, we listen to the popstate event, and when it's fired we keep trying to restore the scroll position, which we only do if the page actually has the correct width and height.
After a few seconds we time out and scroll as far as we can.

Disabling native implementations

While this polyfill is designed to work alongside native implementations, if you nevertheless experience compatibility problems, we suggest you disable native scroll restoration in browsers that support it, like this:

if ('scrollRestoration' in window.history) {
  window.history.scrollRestoration = 'manual';
}

Differences from Chrome's native implementation

We use a timeout instead of checking which resources are loaded and such, because it's more convenient and it doesn't seem to matter much in practice. This does imply that if the user resizes the browser window during their session, causing pages to become less high (e.g. by making the window wider), then we might not restore the scroll position until we time out.
We don't abort restoring the scroll position when the user scrolls, because we cannot detect a user scroll from a browser scroll (e.g. because the page height changes), which the browser can do natively.

Note that we cannot reliably detect if we are in a browser that already supports the delayed scroll restoration behavior, so this polyfill will break Chrome's behavior of aborting scroll restoration on user scroll.
We don't record the scroll position when clicking the back button, only when using history.pushState. This means that we don't get delayed scroll restoration when clicking the forward button in the browser. The reason for this is that we cannot reliably store the scroll position when the back button is clicked, due to spec / browser implementation. See:
This problem can get solved when the Custom Scroll Restoration API gets widely implemented:
- https://code.google.com/p/chromium/issues/detail?id=477353
- http://majido.github.io/scroll-restoration-proposal
The polyfill assumes that on popstate the page is cleared, or at least that the height will only increase right after popstate. We do wait a tick to make sure that your code gets run before we attempt to scroll, but if the page height decreases after that then we might have already scrolled and we won't wait until the page height increases again.

Note that this polyfill just aims to mimic Chrome's behavior as closely as possible, and does not attempt to keep scrolling down in cases when an infinite list is not fully loaded. That case would also break in Chrome, and so you need to provide sufficient caching yourself so that when navigating back to a page it can render again relatively quickly.

Changelog

If you're interested in seeing the changes and bug fixes between each version, read the Changelog.

Code of conduct

This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.

License

This project is released under the MIT license.

janpaul123/delayed-scroll-restoration-polyfill