Quote selected text inside comment reply form

If the user has text selected inside a comment when they click the reply
button, this will automatically start off the form with that text inside
a blockquote. This only works if the selected text is inside another
comment (for example it won't work if the text is in the sidebar or the
topic itself), and only if the entirety of the selection is inside the
same comment.

tildes/static/js/behaviors/comment-reply-button.js

@@ -81,6 +81,27 @@ $.onmount('[data-js-comment-reply-button]', function() {
         replyForm.appendChild(textarea);
         replyForm.appendChild(buttonDiv);
 
+        // If the user has text selected inside a comment when they click the reply
+        // button, start the comment form off with that text inside a blockquote
+        if (window.getSelection) {
+            var selectedText = "";
+
+            // only capture the selected text if it's all from the same comment
+            var selection = window.getSelection();
+            var $start = $(selection.anchorNode).closest(".comment-text");
+            var $end = $(selection.focusNode).closest(".comment-text");
+            if ($start.is($end)) {
+                selectedText = selection.toString();
+            }
+
+            if (selectedText.length > 0) {
+                selectedText = selectedText.replace(/\s+$/g, "");
+                selectedText = selectedText.replace(/^/gm, "> ");
+                textarea.value = selectedText+"\n\n";
+                textarea.scrollTop = textarea.scrollHeight;
+            }
+        }
+
         // update Intercooler so it knows about this new form
         Intercooler.processNodes(replyForm);