--- a/chat/content/convbrowser.xml
+++ b/chat/content/convbrowser.xml
@@ -558,17 +558,18 @@
             if (this._scrollingIntoView) {
               // We have explicitely requested a scrollIntoView, ignore the event
               this._scrollingIntoView = false;
               this._lastScrollHeight = this.scrollHeight;
               this._lastScrollWidth = this.scrollWidth;
               return;
             }
 
-            if (this._lastScrollHeight != this.scrollHeight ||
+            if (!(_lastScrollHeight in this) ||
+                this._lastScrollHeight != this.scrollHeight ||
                 this._lastScrollWidth != this.scrollWidth) {
               // if the scrollheight changed, we are resizing the content area,
               // don't stop the auto scroll.
               this._lastScrollHeight = this.scrollHeight;
               this._lastScrollWidth = this.scrollWidth;
               return;
             }