use a MutationObserver instead, making performance not super terrible

webextension/content-script.js

@@ -1,16 +1,7 @@
 /*
-Unused
-Listen for messages from the page.
-If the message was from the page script, show an alert.
-
-window.addEventListener("message", function(event) {
-  if (event.source == window &&
-      event.data.direction &&
-      event.data.direction == "from-page-script") {
-    alert("Content script received message: \"" + event.data.message + "\"");
-  }
-});
-*/
+ *  Simple script which looks for changes in the chat area, and rolls extra damage
+ *   based on any relevant hit dice.
+ */
 
 function handleMoonslicer(message) {
   var result = message.querySelector('div.sheet-result')
@@ -36,11 +27,11 @@ function handleMoonslicer(message) {
     submit_button.click();
   }
   if (hit0 == 2 || hit1 == 2) {
-    textarea.value = "&{template:npcdmg} {{damage=1}} {{dmg1flag=1}} {{dmg1=[[floor(1d4/2)]] to self}} {{dmg1type=Ice}}";
+    textarea.value = "&{template:npcdmg} {{damage=1}} {{dmg1flag=1}} {{dmg1=[[floor(1d4/2)]]}} {{dmg1type=Ice to self}}}";
     submit_button.click();
   } 
   if (hit0 == 1 || hit1 == 1) {
-    textarea.value = "&{template:npcdmg} {{damage=1}} {{dmg1flag=1}} {{dmg1=[[1d6]] to self}} {{dmg1type=Ice}}";
+    textarea.value = "&{template:npcdmg} {{damage=1}} {{dmg1flag=1}} {{dmg1=[[1d6]]} {{dmg1type=Ice to self}}}";
     submit_button.click();
   }
   textarea.value = old_text;
@@ -49,16 +40,16 @@ function handleMoonslicer(message) {
 /*
 Send a message to the page script.
 */
-function newMessageHandler(event) {
-  var message = event.target;
-  
-  if (message.classList.contains('you')) {
-    var labelElem = message.querySelector('div.sheet-rolltemplate-atk div.sheet-label');
-    var label = labelElem.querySelector('span a');
-    if (label && label.innerHTML == 'Moonslicer') {
+function handleMutation(mutation) {
+  mutation.addedNodes.forEach( function(message) {
+    if (message.classList && message.classList.contains('you')) {
+      var labelElem = message.querySelector('div.sheet-rolltemplate-atk div.sheet-label');
+      var label = labelElem.querySelector('span a');
+      if (label && label.innerHTML == 'Moonslicer') {
         handleMoonslicer(message);
+      };
     };
-  };
+  });
 }
 
 /*
@@ -67,4 +58,7 @@ the "from-content-script" element.
 */
 var textchat = document.getElementById("textchat");
 var content = textchat.getElementsByClassName("content")[0];
-content.addEventListener("DOMNodeInserted", newMessageHandler);
+
+var observer = new MutationObserver(function(mutations) {mutations.forEach(handleMutation);});
+observer.observe(content, {childList: true});
+