"Toggle edit shortcuts" is visible if theme has no styles for `.screen-reader-text` class

[ocean90]

See screenshot from Twenty Twelve.

[westonruter]

@ocean90 Probably would be a good idea to include this in customize-preview.css, yes?

[westonruter]

Deprecate WP_Customize_Manager::customize_preview_loading_style(), moving styles to customize-preview.css. Add screen-reader-text rule.

[westonruter]

Actually, the toggle should have been removed entirely as part of [39131] since the edit shortcuts are now persistent when the controls are expanded.

[sirbrillig]

There's also the translation for that button:

src/wp-includes/customize/class-wp-customize-selective-refresh.php
191:                            'editShortcutVisibilityToggle'  => __( 'Toggle edit shortcuts' ),

[westonruter]

In 39144:

Customize: Remove edit shortcut toggle button from preview since visibility is now linked with pane visibility.

Amends [39131].
Props westonruter, sirbrillig.
See #27403.
Fixes #38668.