[NIFI-2913] update z-index of codemirror resize handle to match the z…

[scottyaslan]

Thank you for submitting a contribution to Apache NiFi.

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there a JIRA ticket associated with this PR? Is it referenced
  in the commit message?
• Does your PR title start with NIFI-XXXX where XXXX is the JIRA number you are trying to resolve? Pay particular attention to the hyphen "-" character.
• Has your PR been rebased against the latest commit within the target branch (typically master)?
• Is your initial contribution a single, squashed commit?

For code changes:

• Have you ensured that the full suite of tests is executed via mvn -Pcontrib-check clean install at the root nifi folder?
• Have you written or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE file, including the main LICENSE file under nifi-assembly?
• If applicable, have you updated the NOTICE file, including the main NOTICE file found under nifi-assembly?
• If adding new Properties, have you added .displayName in addition to .name (programmatic access) for each of the new properties?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

Note:

Please ensure that once the PR is submitted, you check travis-ci for build issues and submit an update to your PR as soon as possible.

…-index of the codemirror vert. and horz. scrollbars

[alopresto]

Reviewing...

[alopresto]

@scottyaslan I built this and once I have entered text that extends past the boundaries to the right and the bottom, I can still use the resize handle, but I can only resize vertically. Is this intentional?

Update: this was on a field which may have been expecting a very small value. I tried on the Replacement Value field of ReplaceText and was able to scroll in both directions after filling the visible text area.

[alopresto]

Ran contrib-check, all tests pass, verified this fixes the issue. +1, merging.