message: Fix highlight of `extend-diff-expander`
diff: @@ -915,8 +915,9 @@ body > .footer li a {
  }
  /* For 'extend-diff-expander' feature */
  -.js-expandable-line:hover .diff-expander,
  -.js-expandable-line:hover .blob-code {
  +/* Hovering the line will highlight the first direction button, unless you hover the buttons directly */
  +.js-expandable-line:hover .blob-num:not(:hover) .directional-expander:first-child,
  +.js-expandable-line:hover .blob-num:not(:hover) + .blob-code {
  border-color: #0366d6;
  color: #fff;
  background: #0366d6;
  