[JENKINS-69359] Code Mirror textbox autosizing is broken

[benebsiny]

See JENKINS-69359.

The original method was to get the html textarea's height and apply it to the codemirror. Now we count the line of the content and the line height and multiply them, then preserve 30px space at the bottom of the codemirror textarea.

Proposed changelog entries

• Fix autosizing of CodeMirror textbox

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change) and are in the imperative mood. Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadoc, as appropriate.
• New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO") if applicable.
• New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content-Security-Policy directives (see documentation on jenkins.io).
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are accurate, human-readable, and in the imperative mood
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[github-actions]

Please take a moment and address the merge conflicts of your pull request. Thanks!

[basil]

@janfaracik Are you interested in reviewing this PR? It resolves a regression from #6259.

[timja]

Tested and works fine for me

[NotMyFault]

Looks reasonable

[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!

[basil]

Manually verified as of commit 3673959