Ignore code block clicks with active selection (#448)

assets/clipboard.js

@@ -11,6 +11,9 @@
 
   document.querySelectorAll("pre code").forEach(code => {
     code.addEventListener("click", function (event) {
+      if (window.getSelection().toString()) {
+        return;
+      }
       select(code.parentElement);
 
       if (navigator.clipboard) {