Remove indiscriminate outline suppression for tabindex="-1" elements (#28437)

instead, only apply outline suppression if the browser wouldn't normally apply the focus outline, using the (currently experimental) `:focus-visible` pseudo-class

Author: patrickhlauke

scss/_reboot.scss

@@ -54,12 +54,16 @@ body {
   background-color: $body-bg; // 2
 }
 
-// Suppress the focus outline on elements that cannot be accessed via keyboard.
-// This prevents an unwanted focus outline from appearing around elements that
-// might still respond to pointer events.
-//
-// Credit: https://github.com/suitcss/base
-[tabindex="-1"]:focus {
+// Future-proof rule: in browsers that support :focus-visible, suppress the focus outline
+// on elements that programmatically receive focus but wouldn't normally show a visible
+// focus outline. In general, this would mean that the outline is only applied if the
+// interaction that led to the element receiving programmatic focus was a keyboard interaction,
+// or the browser has somehow determined that the user is primarily a keyboard user and/or
+// wants focus outlines to always be presented.
+//
+// See https://developer.mozilla.org/en-US/docs/Web/CSS/:focus-visible
+// and https://developer.paciellogroup.com/blog/2018/03/focus-visible-and-backwards-compatibility/
+[tabindex="-1"]:focus:not(:focus-visible) {
   outline: 0 !important;
 }