Accessibility: Fix the Customizer available menu items toggles focus for Safari and VoiceOver.
In rare circumstances, for example buttons with icons and no visible text, user-select: none; may trigger a bug that happens only when using Safari and
VoiceOver and doesn't allow focusable elements to receive focus correctly.
Worth reminding user-select: none; is a non-standard property and should be
used with care especially now that ::selection is almost universally supported.