As far as I know Nintex recommends using Internet Explorer because in some of the other Browsers not all functionality is available.
Hi Yin Hoong,
*Update* You should see that this is resolved now in your designer
This is a defect that the development team is aware of that seems to have been introduced or made worse with the latest release of Chrome (v59). They have a fix for it and it should be rolled out in the next week or so, it's availability for you will depend on your Nintex Data Centre location.
For reference, this help article on browser support does show Chrome (latest) is supported completely.
Hope this helps
Edit: I can't get the link to work, here is the relevant content:
Nintex for Office 365 supports use of the following browsers.
Internet Explorer 9 or 10
Google Chrome, latest release
Firefox, latest release