Clients.showNotification will not show even the target component is in the viewport

Steps to Reproduce

http://zkfiddle.org/sample/3orveup/2-Clients-showNotification-will-not-popup

1. scroll window into the viewport
2. click Popup
3. a popup message is shown

Current Result

The notification will not show on some windows (the 3rd and the last)

Expected Result

All notifications are shown

Root Cause

https://github.com/zkoss/zk/blob/050d1b49d2dfe8e9896cc0a77de5e855e5c2e908/zul/src/archive/web/js/zul/wgt/Notification.js#L247
zk(n).isRealScrollIntoView() returns false, so the notification will not show.

Workaround

Avoid target components that need to scroll into view.

Debug info

Only Safari 11 is not affected.
Chrome 61.0.3163.100, Firefox 55.0.3 and IE9 are all affected.
The notification will not show in the last window in Chrome, but other browsers are correct.