Add tab support

assets/css/style.css

@@ -215,6 +215,7 @@ footer {
   position: relative;
   width: 100%;
 }
+
 @media screen and (max-width:840px) {
   .textareas {
     flex-direction: column;
@@ -282,6 +283,7 @@ footer {
   resize: none;
   width: 100%;
   word-wrap: break-word;
+  tab-size: 4;
 }
 
 @media screen and (max-width:840px) {

assets/js/onload.js

@@ -13,4 +13,21 @@ Start writing!
 `;
   }
   document.getElementById('output').value = math(document.getElementById('input').value);
+
+  // tab support
+  document.getElementById('input').addEventListener('keydown', function (e) {
+    var elem, end, start, value;
+    if (e.keyCode === 9) {
+      if (e.preventDefault) {
+        e.preventDefault();
+      }
+      elem = e.target;
+      start = elem.selectionStart;
+      end = elem.selectionEnd;
+      value = elem.value;
+      elem.value = "" + (value.substring(0, start)) + "\t" + (value.substring(end));
+      elem.selectionStart = elem.selectionEnd = start + 1;
+      return false;
+    }
+  });
 }

index.html

@@ -12,7 +12,6 @@
   <link rel="stylesheet" href="./assets/css/style.css">
   <script src="./assets/js/url.js"></script>  
   <script src="./assets/js/onload.js"></script>
-  <script src="./assets/js/copy.js"></script>
   <script src="./assets/js/math.js"></script>
   <!-- https://docs.mathjax.org/en/latest/web/configuration.html?highlight=heading#configuration-using-an-in-line-script -->
   <script>