Hide the navigation loader when the page is shown

[greg-does-weather]

What does this PR do? 🛠️

When the user clicks the back button, the previous page is not necessarily reloaded. Most of the time, the previous page and its entire state are "popped" from a history stack. There is a a pageshow event that is fired every time a page is shown, however, and that event subscription persists in the history state. By subscribing to that event when page A is first loaded, then when the user navigates to page B and clicks the back button to come back to page A, we can know that page A is – from the user's perspective – reloading. When this event fires, we hide the navigation loader.

Note

This pageshow event is fired after the page is visually restored for the user, so there is a quick flash of the navigation loader before it is hidden. There is an experimental pagereveal event that is fired before visual rendering, but it is not through the standards process yet and is not widely-supported outside of Chrome.

Sometimes the browser will evict a page's state from the browser stack (for example, to reclaim memory if there are too many pages in the stack). In that case, the page is reloaded from scratch. In this case, the navigation loader will be hidden by default.

• fixes Loading indicator persists after clicking back button #1323

[github-actions]

This PR modifies theme Javascript or CSS assets but does not update the theme libraries file. Did you mean to update the appropriate version information in the libraries file?