Fix back-forward cache in rustdoc frontend

[jsha]

Rustdoc's frontend set a no-op unload handler, specifically to disable
Firefox's back-forward cache because it caused a bug. It's nice to
allow the back-forward cache because it permits faster navigations.

This change addresses the issues that were caused by back-forward cache.

https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/1.5/Using_Firefox_1.5_caching
https://web.dev/bfcache/

Demo: https://jacob.hoffman-andrews.com/rust/fix-bfcache/std/string/struct.String.html

Related: #72272

[rust-highfive]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @ollie27 (or someone else) soon.

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[jsha]

There's another, somewhat simpler fix that eliminates the need for any new JS: We can just remove the autocomplete="off" property on the search-input. I've tested this and it works beautifully.

According to https://developer.mozilla.org/en-US/docs/Web/Security/Securing_your_site/Turning_off_form_autocompletion:

Setting autocomplete="off" on fields has two effects:

• It tells the browser not to save data inputted by the user for later autocompletion on similar forms, though heuristics for complying vary by browser.
• It stops the browser from caching form data in the session history. When form data is cached in session history, the information filled in by the user is shown in the case where the user has submitted the form and clicked the Back button to go back to the original form page.

That second bullet point explains why we had this bad interaction with the bfcache. Normally, the input field would be preserved across forward/back navigations. But because it has autocomplete="off", it was empty on returning to the page. Rustdoc's JS saw the empty input and treats that as a signal to clear the search.

[GuillaumeGomez]

To be noted that the search function is now called twice by default on any page (except that it does nothing since there is no search query). It's really minor and I think that the improvement itself is definitely better!

Thanks a lot @jsha !

@bors: r+

[bors]

📌 Commit 51a14c7 has been approved by GuillaumeGomez

[bors]

⌛ Testing commit 51a14c7 with merge 0846043...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing 0846043 to master...