Our team is working to reduce flicker as much as possible in our test activities, and are weighing the options for using a Custom Global Mbox (current implementation) versus auto-creating a Global Mbox.
Having said that, if you are creating VEC activities, you must have Global MBOX Autocreate enabled in at.js to ensure content is delivered correctly to the page. In general we do not suggest to have any customization done to the at.js library, so it is best to test with an at.js with Auto create Global mbox.
I believe our flicker is a result of using a custom global mbox. We prevent flicker by setting the targeted content to '0' opacity, then change the opacity to '1' once Target fires--similar to what the Auto Created Global Mbox does, just not on the body.
So, a few final questions:
Does using Global Auto Create, force us to build test activities with the VEC?
Are they mutually exclusive?
Will using the Form-based Composer have any affect on flicker?