[JENKINS-71252][JENKINS-70793] Multiple form validation fixes

[daniel-beck]

See JENKINS-71252 and JENKINS-70793.

(The fix for JENKINS-70793 only works with the fix for JENKINS-71252.)

JENKINS-71252

Per https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/offsetHeight,

If the element is hidden (for example, by setting style.display on the element or one of its ancestors to "none"), then 0 is returned.

Requesting review by @janfaracik to understand whether there's any reason to use this over auto.

JENKINS-70793

https://github.com/jenkinsci/jenkins/pull/6460/files#diff-10922aee3260853be495a09cf3bf13bf5f931bc128e334807e3008ff00b57651R523-R528 looked sus, so I deleted it.

https://github.com/jenkinsci/jenkins/pull/6460/files#r862952504 looks a little concerning, but I was unable to reproduce that with sample plugins. Requesting @timja review just in case.

Testing done

Followed the steps to reproduce in my Jira comment to JENKINS-71252.

Installed @yaroslavafenkin's demo plugin for JENKINS-71252 and confirmed it behaved correctly.

Patched the demo plugin JENKINS-71252 to have a stack trace like described in JENKINS-70793, and tested.

Proposed changelog entries

• Show form validation results for form elements that are initially hidden. (regression in 2.355)
• Remove previous form validation errors when the form validation is updated with new content. (regression in 2.355)

Proposed upgrade guidelines

N/A

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@mention

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[daniel-beck]

This line and following are probably obsolete now and can be deleted?

[MarkEWaite]

@janfaracik we'll select the next LTS baseline this week. I think this looks like a very promising fix to consider including in Jenkins 2.426 weekly so that it can be included in the Nov 15, 2023 LTS release. Would you be willing to review it?

[timja]

Looks sensible

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!