Improve overflow/underflow DOM event handling. b=365477 r+sr=roc

browser/base/content/tabbrowser.xml

@@ -2547,6 +2547,11 @@
         if (event.target != this)
           return;
 
+        // Ignore vertical events.
+        if (event.detail == 0) {
+          return;
+        }
+
         this._scrollButtonDownBox.collapsed = true;
         this._scrollButtonDownBoxAnimate.collapsed = true;
       ]]></handler>
@@ -2556,6 +2561,11 @@
         if (event.target != this)
           return;
 
+        // Ignore vertical events.
+        if (event.detail == 0) {
+          return;
+        }
+
         this._scrollButtonDownBox.collapsed = false;
         this._scrollButtonDownBoxAnimate.collapsed = false;
       ]]></handler>