shuffle the items before they are rendered and displayed
Do the items have any JavaScript event listeners assigned to them. If not (meaning if the 'container' only contains markup and no script dependency) then I would suggest:
- creating an array that stores the markup of each individual masonry-item as HTML string.
- Shuffle the array and
- dump the array contents into the 'container'
A crude solution for sure. But, hope this gets the job done.