fix(*): change fallback font to sans and optimize back to top

source/css/style.styl

@@ -1,4 +1,4 @@
-family-serif = "Ubuntu", "Roboto", "Open Sans", "Microsoft YaHei", serif
+family-sans = "Ubuntu", "Roboto", "Open Sans", "Microsoft YaHei", sans-serif
 family-mono = "Source Code Pro", monospace, "Microsoft YaHei"
 
 /* ---------------------------------
@@ -8,7 +8,7 @@ html
     font-size: 14px
 
 body
-    font-family: family-serif
+    font-family: family-sans
     background-color: #f7f7f7
 
 @media screen and (min-width: 1280px)

source/js/back-to-top.js

@@ -7,7 +7,7 @@ $(document).ready(function () {
     var lastScrollTop = 0;
     var rightMargin = 20;
     var bottomMargin = 20;
-
+    var lastState = null;
     var state = {
         base: {
             classname: 'card has-text-centered',
@@ -36,7 +36,24 @@ $(document).ready(function () {
         classname: state['mobile-hidden'].classname + ' rise-up',
     });
 
+    function isStateEquals(prev, next) {
+        for (var prop in prev) {
+            if (!next.hasOwnProperty(prop) || next[prop] !== prev[prop]) {
+                return false;
+            }
+        }
+        for (var prop in next) {
+            if (!prev.hasOwnProperty(prop) || prev[prop] !== prev[prop]) {
+                return false;
+            }
+        }
+        return true;
+    }
+
     function applyState(state) {
+        if (lastState !== null && isStateEquals(lastState, state)) {
+            return;
+        }
         $button.attr('class', state.classname);
         for (let prop in state) {
             if (prop === 'classname') {
@@ -44,6 +61,7 @@ $(document).ready(function () {
             }
             $button.css(prop, state[prop]);
         }
+        lastState = state;
     }
 
     function isDesktop() {