Use CSS for hiding thread watcher close button.

src/General/css/style.css

@@ -837,6 +837,9 @@ span.hide-announcement {
 #thread-watcher a {
   text-decoration: none;
 }
+:root:not(.fixed-watcher) #thread-watcher .move > .close {
+  display: none;
+}
 #thread-watcher .move > .close {
   position: absolute;
   right: 0px;

src/Monitoring/ThreadWatcher.coffee

@@ -18,7 +18,6 @@ ThreadWatcher =
     @closeButon = $('.move > .close', @dialog)
     @unreaddb = Unread.db or new DataBoard 'lastReadPosts'
 
-    @closeButon.hidden = not Conf['Toggleable Thread Watcher']
     if Conf['Toggleable Thread Watcher']
       @setHidden Conf['Thread Watcher Hidden']
       $.sync 'Thread Watcher Hidden', @setHidden