src/macosx/classes/sun/lwawt/LWWindowPeer.java

@@ -801,10 +801,14 @@
         if (targetPeer == null || !targetPeer.isEnabled()) {
             return;
         }
 
         Point lp = targetPeer.windowToLocal(x, y, this);
+        int signum = (int) Math.signum(preciseWheelRotation);
+        if (wheelRotation == 0 && signum * wheelRotation < 1) {
+           wheelRotation = signum;
+        }
         // TODO: fill "bdata" member of AWTEvent
         // TODO: screenX/screenY
         postEvent(new MouseWheelEvent(targetPeer.getTarget(),
                                       MouseEvent.MOUSE_WHEEL,
                                       when, modifiers,