Don't open the QR in closed threads. Ideally I'd make the QR independant of the DOM but it's impossible without captcha and known board configs.

4chan_x.user.js

@@ -2832,7 +2832,7 @@
       return key;
     },
     qr: function(thread, quote) {
-      if (!Conf['Quick Reply']) {
+      if (!(Conf['Quick Reply'] && QR.postingIsEnable)) {
         return;
       }
       QR.open();
@@ -5641,7 +5641,6 @@
 
   QR = {
     init: function() {
-      var link;
       if (g.VIEW === 'catalog' || !Conf['Quick Reply']) {
         return;
       }
@@ -5650,6 +5649,18 @@
       if (Conf['Hide Original Post Form']) {
         $.addClass(doc, 'hide-original-post-form');
       }
+      $.on(d, '4chanXInitFinished', this.initReady);
+      return Post.prototype.callbacks.push({
+        name: 'Quick Reply',
+        cb: this.node
+      });
+    },
+    initReady: function() {
+      var link;
+      QR.postingIsEnable = !!$.id('postForm');
+      if (!QR.postingIsEnable) {
+        return;
+      }
       link = $.el('a', {
         className: 'qr-shortcut',
         textContent: 'Quick Reply',
@@ -5669,9 +5680,6 @@
       $.on(d, 'dragover', QR.dragOver);
       $.on(d, 'drop', QR.dropFile);
       $.on(d, 'dragstart dragend', QR.drag);
-      if (Conf['Persistent QR']) {
-        $.on(d, '4chanXInitFinished', QR.persist);
-      }
       $.on(d, 'ThreadUpdate', function() {
         if (g.DEAD) {
           return QR.abort();
@@ -5679,10 +5687,9 @@
           return QR.status();
         }
       });
-      return Post.prototype.callbacks.push({
+      if (Conf['Persistent QR']) {
-        name: 'Quick Reply',
+        return QR.persist();
-        cb: this.node
+      }
-      });
     },
     node: function() {
       return $.on($('a[title="Quote this post"]', this.nodes.info), 'click', QR.quote);
@@ -5922,6 +5929,9 @@
       if (e != null) {
         e.preventDefault();
       }
+      if (!QR.postingIsEnable) {
+        return;
+      }
       text = "";
       sel = d.getSelection();
       selectionRoot = $.x('ancestor::div[contains(@class,"postContainer")][1]', sel.anchorNode);

src/features.coffee

@@ -1554,7 +1554,7 @@ Keybinds =
     key
 
   qr: (thread, quote) ->
-    return unless Conf['Quick Reply']
+    return unless Conf['Quick Reply'] and QR.postingIsEnable
     QR.open()
     if quote
       QR.quote.call $ 'input', $('.post.highlight', thread) or thread

src/qr.coffee

@@ -8,6 +8,16 @@ QR =
     if Conf['Hide Original Post Form']
       $.addClass doc, 'hide-original-post-form'
 
+    $.on d, '4chanXInitFinished', @initReady
+
+    Post::callbacks.push
+      name: 'Quick Reply'
+      cb:   @node
+
+  initReady: ->
+    QR.postingIsEnable = !!$.id 'postForm'
+    return unless QR.postingIsEnable
+
     link = $.el 'a',
       className: 'qr-shortcut'
       textContent: 'Quick Reply'
@@ -25,16 +35,13 @@ QR =
     $.on d, 'dragover',           QR.dragOver
     $.on d, 'drop',               QR.dropFile
     $.on d, 'dragstart dragend',  QR.drag
-    $.on d, '4chanXInitFinished', QR.persist if Conf['Persistent QR']
     $.on d, 'ThreadUpdate', ->
       if g.DEAD
         QR.abort()
       else
         QR.status()
 
-    Post::callbacks.push
+    QR.persist() if Conf['Persistent QR']
-      name: 'Quick Reply'
-      cb:   @node
 
   node: ->
     $.on $('a[title="Quote this post"]', @nodes.info), 'click', QR.quote
@@ -229,6 +236,7 @@ QR =
 
   quote: (e) ->
     e?.preventDefault()
+    return unless QR.postingIsEnable
     text = ""
 
     sel = d.getSelection()