Add copy to clipboard buttons to code blocks

Only load the JS on posts that actually have code blocks.

layouts/blog/single.html

@@ -37,3 +37,10 @@ <h1>{{ .Title }}</h1>
 
     <div class="separator">
 {{ end }}
+
+{{ define "js-extra" }}
+    {{ if (findRE "class=\"highlight\"" .Content 1) }}
+        <script src="https://cdnjs.cloudflare.com/ajax/libs/clipboard-polyfill/2.7.0/clipboard-polyfill.promise.js" integrity="sha256-waClS2re9NUbXRsryKoof+F9qc1gjjIhc2eT7ZbIv94=" crossorigin="anonymous"></script>
+        <script src="/js/copy-code-button.js"></script>
+    {{ end  }}
+{{ end  }}

static/css/post.css

@@ -24,3 +24,38 @@ h1 {
   text-align: right;
   width: 48%;
 }
+
+.copy-code-button {
+  color: #272822;
+  background-color: #FFF;
+  border-color: #272822;
+  border: 2px solid;
+  border-radius: 3px;
+
+  /* right-align */
+  display: block;
+  margin-left: auto;
+  margin-right: 0;
+
+  /* for some reason, there is space between the button and the code block */
+  margin-bottom: -10px;
+
+  padding: 3px 8px;
+  font-size: 0.8em;
+}
+
+.copy-code-button:hover {
+  cursor: pointer;
+  background-color: #F2F2F2;
+}
+
+.copy-code-button:focus {
+  /* avoid an ugly focus outline on click in Chrome, but darken the button
+     for accessibility. https://stackoverflow.com/a/25298082/1481479 */
+  background-color: #E6E6E6;
+  outline: 0;
+}
+
+.copy-code-button:active {
+  background-color: #D9D9D9;
+}

static/js/copy-code-button.js

@@ -0,0 +1,26 @@
+if (clipboard) {
+    document.querySelectorAll('.highlight').forEach(function (codeBlock) {
+        var button = document.createElement('button');
+
+        button.className = 'copy-code-button';
+        button.type = 'button';
+        button.innerText = 'Copy';
+
+        button.addEventListener('click', function () {
+            clipboard.writeText(codeBlock.innerText).then(function () {
+                /* Chrome doesn't seem to blur automatically, leaving the button
+                   in a focused state */
+                button.blur();
+
+                button.innerText = 'Copied!';
+                setTimeout(function () {
+                    button.innerText = 'Copy';
+                }, 2000);
+            }, function (error) {
+                button.innerText = 'Error';
+            });
+        });
+
+        codeBlock.parentNode.insertBefore(button, codeBlock);
+    });
+}