[3.x] Only reset useForm processing and progress with onFinish

[mattwigham]

Closes #2463

Previously the form helper's onSuccess, onError, onCancel, and onFinish callbacks all set processing = false and progress = null. This PR makes it so that now only happens in onFinish.

Aside from the pedantic and DRY reasons to do this, this also helps those of us using the feature of onSuccess and onError where you can return a promise, and onFinish won't be called until it resolves.

I'm using that feature in some forms that do work in a background job, which I poll the status of inside the promise, and this change allows the form's processing to remain true during that entire time and not just the initial submission.

[pascalbaljet]

This is technically a breaking change. Code that checks form.processing inside onSuccess/onError would now see true instead of false. For example, if you're showing a spinner based on processing within those callbacks, it would still be visible until onFinish fires.

That said, I think this change makes sense since it aligns with the documented promise behavior. I'll merge this in v3.

[pascalbaljet]

@mattwigham Can you tick the 'Allow edits from maintainers' checkbox so I can add more tests? Thanks!

[mattwigham]

@pascalbaljet I just realized this is from our gh org, so I don't have that option to allow edits. Would you like me to resubmit the PR as a user or do you want to recreate these changes yourself since they're minimal? Sorry for the hassle!

[pascalbaljet]

@pascalbaljet I just realized this is from our gh org, so I don't have that option to allow edits. Would you like me to resubmit the PR as a user or do you want to recreate these changes yourself since they're minimal? Sorry for the hassle!

No problem, I'll open a new PR and add you as co-author!