Not sure what you mean with "contextmenu". If you mean this speech bubble thing with the specs:
You can click on the Popover on button above the rack to prevent the display of the popover.

It's basically the right-click menu. I activates on mobiles when you long-press a module, just like the popover, which it obscures. You can get around it if you drag really fast, but that usually just moves an unintended module. My standard procedure is to press a module, wait for the contextmenu to open, press somewhere else on the screen to close the contextmenu, then drag the module. I spend way more time on ModularGrid than I should already, so every second counts. I believe you can get around it by something like:

if(/Android|iPhone|iPad/i.test(navigator.userAgent)) {
    window.addEventListener('DOMContentLoaded', function() {
        document.querySelector('#rack').addEventListener('contextmenu', function(e) {
            if(e.target.localName === 'img') {
                e.preventDefault();
            }
        });
    });
}

Thanks again for all the awesomeness.