disable Bootstrap's Collapse open/close animation

For Bootstrap 3 and 4 it's

.collapsing {
    -webkit-transition: none;
    transition: none;
    display: none;
}

Bootstrap 2 CSS solution:

.collapse {  transition: height 0.01s; }  

NB: setting transition: none disables the collapse functionnality.

---

Bootstrap 4 solution:

.collapsing {
  transition: none !important;
}

If you find the 1px jump before expanding and after collapsing when using the CSS solution a bit annoying, here's a simple JavaScript solution for Bootstrap 3...

Just add this somewhere in your code:

$(document).ready(
    $('.collapse').on('show.bs.collapse hide.bs.collapse', function(e) {
        e.preventDefault();
    }),
    $('[data-toggle="collapse"]').on('click', function(e) {
        e.preventDefault();
        $($(this).data('target')).toggleClass('in');
    })
);