[wp-trac] [WordPress Trac] #35649: Customizer: spinner animated GIF and high CPU usage

[WordPress Trac]

#35649: Customizer: spinner animated GIF and high CPU usage
-----------------------------+-----------------
 Reporter:  afercia          |      Owner:
     Type:  defect (bug)     |     Status:  new
 Priority:  normal           |  Milestone:  4.5
Component:  Customize        |    Version:
 Severity:  normal           |   Keywords:
  Focuses:  ui, performance  |
-----------------------------+-----------------
 As a best practice and as already done for the Theme Installer and the
 ThickBox modal, see tickets [ticket:33322 33322] and [ticket:33311 33311],
 when the spinner animated GIF is used as a background image, the element
 the background is applied to should be
 [https://developers.google.com/web/fundamentals/performance/rendering
 /simplify-paint-complexity-and-reduce-paint-areas#promote-elements-that-
 move-or-fade promoted to a new layer] and
 [https://developers.google.com/web/fundamentals/performance/rendering
 /simplify-paint-complexity-and-reduce-paint-areas#promote-elements-that-
 move-or-fade the re-painted area should be reduced] to the smallest
 possible one.

 `#customize-container` and `#customize-preview` should just reuse the same
 CSS used for the Theme Installer.

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Ticket URL: <https://core.trac.wordpress.org/ticket/35649>
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