Fix form validation with js disabled

[talldan]

Fixes part of #41942

Proposed changes:

Fixes a bug where, if the user has JavaScript disabled in their browser, and tries submitting an invalid form, the user ends up on a 404 page instead of staying on the same page with form validation errors shown.

This is only a partial fix, there's still a bug mentioned in the comments on the issue, where I also mention how it seems to have been introduced - #41942 (comment).

The remaining bug is that the user needs to submit the invalid form twice for the validation issues to show. It's not great, the first time submitting the form will be reset. The fix is probably going to be quite considerable, or involve reverting the PR mentioned in the comment, but that would bring back another (much worse IMO) bug.

I expect this is very low priority though, most users won't have JS disabled, so I don't plan to work on it further than this.

Other information:

• Have you written new tests for your changes, if applicable?
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Does this pull request change what data or activity we track or use?

No

Testing instructions:

1. Add a form to a post
2. Make sure the form has at least one required field
3. Save and view the post
4. Disable JS in your browser and reload the post
5. Submit the form

In trunk: The user ends up on a 404 page
In this PR: The page reloads

[github-actions]

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[github-actions]

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[talldan]

Fix is to only add the ?page query param to the form submission url if the post has more than one page. The $multipage global stores whether it's a multi page post with a value of 0 or 1 so we can use that to check.

Perhaps in classic themes it was possible for users to navigate to a URL like that and not end up on a 404 page, but it doesn't seem to be the case in block themes.

[jp-launch-control]

Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

Full summary · PHP report · JS report