Double click prevention

[laryn]

Description of the need

I was working on a form that has a bit of a lag as it checks something via an external API, and started looking into double click prevention. It turns out it was added to Drupal 7 in 7.98 -- should we add it to Backdrop as well?

• Change record: https://www.drupal.org/node/3365142
• Issue: https://www.drupal.org/node/1705618

[klonos]

I thought we already sorted that with #2595 🤔 ...there's also #3717 as a follow-up.

[klonos]

...right, it seems that we are missing a few things from this commit, plus also this one which introduces the new drupal.form-single-submit library.

So yeah, we should do that 👍🏼 ...but if we brought code parity with these commit, then I believe that we can close #3717 as redundant(?).

[klonos]

...I'm assigning this to myself, to knock it out of the way.

[klonos]

So the way things were implemented in D7, the JS that does the double-click prevention was added into a separate file, loaded via the system_library(), whereas we have added it to the existing form.js file in Backdrop.

Also, in D7.36 a javascript_always_use_jquery setting was introduced (change record), which was before this change here. So the equivalent commit includes tests that take that setting into consideration as well. Plus, the tests are checking if misc/form-single-submit.js is loaded on the page when a form exists, but in our case where the code lives in form.js, it will always be loaded, so the tests don't make sense or any point as they were implemented in D7.

So questions:

• do we also introduce javascript_always_use_jquery setting or not? (see [PS] jQuery and Backdrop JavaScript libraries and settings always output #3669 and Allow the 'javascript_always_use_jquery' variable to be configured via the Performance page #3671)
• if we do introduce the javascript_always_use_jquery setting, do we wait for that before making this change here?
• do we split Backdrop.behaviors.formSingleSubmit out of the form.js file and load it separately via system_library() or leave it as is in form.js?

[herbdool]

@klonos I don't think we need to wait, and I think it makes sense that the test won't work with the way Backdrop implemented it. So I think the PR is ready for review and testing. The question I have is, what's the best way to manually test to see if double click prevention is working and if it can be disabled by setting javascript_use_double_submit_protection to TRUE?