Currently, the "Collapse menu" item is not focusable and keyboard users can't collapse/expand the admin menu. This aims to fix it so that screen readers no longer announce it as a clickable but it remains unfocusable and thus unusable. So it's now a button.
Quoting joedolson at WordCamp Chicago 2014:
"If it's supposed to act like a button, it should be a button."
Also includes a grunt:precommit run that picked up some postcss changes to src/wp-includes/css/customize-preview.css
Fixes#29958.
Props ajercia, ipm-frommen for an iterative patch, valendesigns for an iterative patch, GaryJ for feedback, joedolson for feedback, helen for feedback
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