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public/admin/view/stylesheet/sass/bootstrap/mixins/_tab-focus.scss

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+// WebKit-style focus
+
+@mixin tab-focus() {
+  // WebKit-specific. Other browsers will keep their default outline style.
+  // (Initially tried to also force default via `outline: initial`,
+  // but that seems to erroneously remove the outline in Firefox altogether.)
+  outline: 5px auto -webkit-focus-ring-color;
+  outline-offset: -2px;
+}