[wp-trac] [WordPress Trac] #28661: Improve loading of Customizer from front-end by leveraging `customize-loader.js`

[WordPress Trac]

#28661: Improve loading of Customizer from front-end by leveraging `customize-
loader.js`
------------------------------+-----------------------------
 Reporter:  celloexpressions  |      Owner:
     Type:  enhancement       |     Status:  new
 Priority:  normal            |  Milestone:  Awaiting Review
Component:  Customize         |    Version:  3.4
 Severity:  normal            |   Keywords:
  Focuses:  ui, javascript    |
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 The Customizer loads "instantly" from the themes screen via `customize-
 loader.js` (loading via an iframe and displaying an animation as soon as
 it starts loading). This also means that the customizer can be exited
 ''instantly''. With #25457, we're looking at doing this throughout the
 admin.

 It turns out, we can also load the customizer more seamlessly from the
 front-end with `customize-loader`. It's just a matter of enqueuing the
 necessary (<10kb) resources for users who can customize and adding the
 customize-loader class.

 This is a good first step that would enable us to better consider:
 - Making the customize link more prominent in the admin bar (front-end and
 back-end, potentially), with a top-level text and icon placement.
 - Changing the customize-load animation so that the controls slide-in, or
 similar, to better show the purpose of the customizer's front-end context
 (along with #28655).
 - Improving `customize-loader` on the front-end by showing the existing
 page DOM in place of the iframe until the iframe loads (making it feel
 much quicker).

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Ticket URL: <https://core.trac.wordpress.org/ticket/28661>
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