Restore flexbox for horizontal panel header titles.

[TomTirapani]

Hoist P/R Checklist

Pull request authors: Review and check off the below. Items that do not apply can also be
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• Caught up with develop branch as of last change.
• Added CHANGELOG entry, or determined not required.
• Reviewed for breaking changes, added breaking-change label + CHANGELOG if so.
• Updated doc comments / prop-types, or determined not required.
• Reviewed and tested on Mobile, or determined not required.
• Created Toolbox branch / PR, or determined not required.

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