The lack of border is because we use arrowpanel-border-color, while we used to use --toolbar-field-focus-border-color https://searchfox.org/mozilla-central/rev/759872688df15a5d6ab305ffe39d90450590bfec/browser/themes/shared/urlbar-searchbar.inc.css#137
Bug 1703590 Comment 5 Edit History
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The lack of border is because we use arrowpanel-border-color, while we used to use --toolbar-field-focus-border-color https://searchfox.org/mozilla-central/rev/759872688df15a5d6ab305ffe39d90450590bfec/browser/themes/shared/urlbar-searchbar.inc.css#137 Again, here we are more consistent with the other panels, indeed if you open the hamburger menu with that theme, it doesn't have a visible border. We could revert, with a consistency cost.