[wp-trac] [WordPress Trac] #38585: Exit `wp-custom-header.js` gracefully in browsers without `addEventListener` support.

[WordPress Trac]

#38585: Exit `wp-custom-header.js` gracefully in browsers without
`addEventListener` support.
---------------------------+-----------------
 Reporter:  peterwilsoncc  |      Owner:
     Type:  defect (bug)   |     Status:  new
 Priority:  normal         |  Milestone:  4.7
Component:  Themes         |    Version:
 Severity:  normal         |   Keywords:
  Focuses:  javascript     |
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 `wp-custom-header.js` throws an error in browsers that don't support
 `document.addEventListener`.

 Add a check for browser support and fail gracefully if it doesn't pass.

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Ticket URL: <https://core.trac.wordpress.org/ticket/38585>
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