I haven't checked for regression range but the code responsible for removing "open" attribute from the button (which is how CSS know when to style it as focused) in this case was added in bug 1701664.
Bug 1702125 Comment 1 Edit History
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I haven't checked for regression range but the code responsible for removing "open" attribute from the button (which is how CSS know when to style it as focused) in this case was added in bug 1701664 here: https://searchfox.org/mozilla-central/rev/9043e515e9608cc55b252a40cb2dfb6f767bcffd/toolkit/content/widgets/panel.js#233-237