Improve the smooth scrolling

- Add smooth scrolling to the footnotes
- Add hash to URL

assets/css/_addon/main.scss

@@ -671,27 +671,19 @@ kbd {
   margin: 0 0.3rem;
 }
 
-sup {
-  z-index: 1;
-  &:target {
-    @extend %anchor;
-    z-index: 0;
-  }
-}
-
 .footnotes > ol {
   padding-left: 2rem;
   margin-top: 0.5rem;
   > li {
-    + li {
-      margin-top: 0.3rem;
+    &:not(:last-child) {
+      margin-bottom: 0.3rem;
     }
     > p {
       margin-left: 0.25em;
       margin-top: 0;
       margin-bottom: 0;
     }
-    &:target > p {
+    &:target:not([scroll-focus]), &[scroll-focus=true] > p { // [scroll-focus] added by `smooth-scroll.js`
       background-color: var(--footnote-target-bg);
       width: fit-content;
       -webkit-transition: background-color 1.5s ease-in-out;
@@ -703,14 +695,14 @@ sup {
 
 .footnote {
   @at-root a#{&} {
-    margin: 0 0.2em;
+    @include ml-mr(1px);
+    @include pl-pr(2px);
     border-bottom-style: none !important;
     -webkit-transition: background-color 1.5s ease-in-out; /* Safari prior 6.1 */
     transition: background-color 1.5s ease-in-out;
   }
-  @at-root sup:target > a#{&} {
+  @at-root sup:target:not([scroll-focus]), sup[scroll-focus=true] > a#{&} { // [scroll-focus] added by `smooth-scroll.js`
     background-color: var(--footnote-target-bg);
-    padding: 0 2px;
   }
 }
 
@@ -1204,13 +1196,9 @@ img {
     justify-content: center!important;
   }
 
-  sup:target {
-    padding-top: 3.4rem;
-  }
-
   .footnotes ol > li {
     padding-top: 3.5rem;
-    margin-top: -3.2rem !important;
+    margin-top: -3.2rem;
     &:first-child {
       margin-top: -3.5rem;
     }

assets/js/_utils/smooth-scroll.js

@@ -16,18 +16,56 @@ $(function() {
       if (location.pathname.replace(/^\//, "") === this.pathname.replace(/^\//, "")
         && location.hostname === this.hostname) {
 
-      var target = $(decodeURI(this.hash));
+        const REM = 16; /* 16px */
+
+        const hash = decodeURI(this.hash);
+        let isFnRef = RegExp(/^#fnref:/).test(hash);
+        let isFn = RegExp(/^#fn:/).test(hash);
+        let selector = hash.includes(":") ? hash.replace(/\:/, "\\:") : hash;
+        const target = $(selector);
 
         if (target.length) {
-
           event.preventDefault();
 
-        $("html, body").animate({
-          scrollTop: target.offset().top
-        }, 800, function() {
+          if (history.pushState) { /* add hash to URL */
+            history.pushState(null, null, hash);
+          }
+
+          let curOffset = $(this).offset().top;
+          let destOffset = target.offset().top;
+          const scrollUp = (destOffset < curOffset);
+          const topbarHeight = $("#topbar-wrapper").outerHeight();
+
+          if (scrollUp && isFnRef) {
+            /* Avoid the top-bar covering `fnref` when scrolling up
+              because `fnref` has no `%anchor`(see: module.scss) style. */
+            destOffset -= (topbarHeight + REM / 2);
+          }
+
+          $("html,body").animate({
+            scrollTop: destOffset
+          }, 800, () => {
+
             var $target = $(target);
             $target.focus();
 
+            const SCROLL_MARK = "scroll-focus";
+
+            /* clean up old scroll mark */
+            if ($(`[${ SCROLL_MARK }=true]`).length) {
+              $(`[${ SCROLL_MARK }=true]`).attr(SCROLL_MARK, false);
+            }
+
+            /* Clean :target links */
+            if ($(":target").length) { /* element that visited by the URL with hash */
+              $(":target").attr(SCROLL_MARK, false);
+            }
+
+            /* set scroll mark to footnotes */
+            if (isFn || isFnRef) {
+              $target.attr(SCROLL_MARK, true);
+            }
+
             if ($target.is(":focus")) { /* Checking if the target was focused */
               return false;
             } else {