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

@@ -13,30 +13,68 @@ $(function() {
     .not("[href='#0']")
     .click(function(event) {
 
-    if (location.pathname.replace(/^\//, "") === this.pathname.replace(/^\//, "")
-      && location.hostname === this.hostname) {
+      if (location.pathname.replace(/^\//, "") === this.pathname.replace(/^\//, "")
+        && location.hostname === this.hostname) {
 
-      var target = $(decodeURI(this.hash));
+        const REM = 16; /* 16px */
 
-      if (target.length) {
+        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);
 
-        event.preventDefault();
+        if (target.length) {
+          event.preventDefault();
 
-        $("html, body").animate({
-          scrollTop: target.offset().top
-        }, 800, function() {
-          var $target = $(target);
-          $target.focus();
-
-          if ($target.is(":focus")) { /* Checking if the target was focused */
-            return false;
-          } else {
-            $target.attr("tabindex", "-1"); /* Adding tabindex for elements not focusable */
-            $target.focus(); /* Set focus again */
+          if (history.pushState) { /* add hash to URL */
+            history.pushState(null, null, hash);
           }
-        });
-      }
-    }
 
-  }); /* click() */
+          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 {
+              $target.attr("tabindex", "-1"); /* Adding tabindex for elements not focusable */
+              $target.focus(); /* Set focus again */
+            }
+          });
+        }
+      }
+
+    }); /* click() */
 });