Add transparent pseudo-element for border

Author: sabernhardt

src/wp-includes/css/buttons.css

@@ -194,6 +194,7 @@ TABLE OF CONTENTS:
 	color: var(--wp-admin-theme-color-darker-20, #183ad6);
 	border-color: var(--wp-admin-theme-color, #3858e9);
 	box-shadow: inset 0 2px 6px -2px  var(--wp-admin-theme-color-darker-20);
+	position: relative;
 }
 
 .wp-core-ui .button.active:focus {
@@ -203,6 +204,19 @@ TABLE OF CONTENTS:
 	box-shadow: inset 0 2px 6px -2px  var(--wp-admin-theme-color-darker-20), 0 0 0 var(--wp-admin-border-width-focus, 1.5px) var(--wp-admin-theme-color, #3858e9);
 }
 
+/* Only visible in Windows High Contrast mode */
+.wp-core-ui .button.active:before {
+	content: "";
+	display: block;
+	position: absolute;
+	width: 100%;
+	height: 100%;
+	border: 1px solid transparent;
+	border-bottom-width: 3px;
+	left: 0;
+	box-sizing: border-box;
+}
+
 .wp-core-ui .button[disabled],
 .wp-core-ui .button:disabled,
 .wp-core-ui .button.disabled,