Flexbox Performance in Safari for Sticky Sidebar - css

Question

I'm looking to display a sticky sidebar on my page, positioned relatively when the client is at the top of the document and fixed when the client scrolls.
My problem is with Safari (it seems to work fine with Chrome and Firefox).
I had it working with a set of floating divs (sidebar floats left) but when I changed to flexbox I found that fast scrolling was too fast for the sticky sidebar to keep up (whether detaching itself from its relative position or staying in a fixed position on the page.
Has anyone else encountered a similar issue and found a work around? I'll go back to floats if necessary but as I'm still learning this stuff it would be great to get to the bottom of why flexbox is causing problems.
Thanks! (Code/markup follows)
HTML:
<div class="order-form">
<div class="left-column" id="left-column">
<div class="sidebar" id="sidebar">
<ul id="courses"></ul>
</div>
</div>
<div class="menu" id="menu"><!-- content --></div>
<div class="right-column"> <!-- content --></div>
</div>
JQuery:
$(window).scroll(function() {
stickers();
});
function stickers() {
var sidebar = $(".sidebar");
if ($(window).scrollTop() > 300) {
sidebar.addClass("scrolling-sidebar");
} else {
sidebar.removeClass("scrolling-sidebar");
}
}
Old, unproblematic, CSS:
.order-form {
padding: 0 300px 0 120px;
}
.sidebar {
width: 150px;
font-weight: lighter;
text-transform: uppercase;
display: block;
}
.sidebar.scrolling-sidebar {
position: fixed;
top: 75px;
left: 5%
}
My current CSS file (compiled from Sass, with problems seemingly arising from my use of flexbox):
.order-form {
margin: 50px 0;
padding: 0 5%;
display: -webkit-box;
display: -webkit-flex;
display: -moz-flex;
display: -ms-flexbox;
display: flex;
-webkit-align-items: stretch;
align-items: stretch;
-webkit-flex-direction: row;
-moz-flex-direction: row;
flex-direction: row;
-webkit-flex-wrap: nowrap;
-moz-flex-wrap: nowrap;
flex-wrap: nowrap;
-webkit-justify-content: flex-start;
-moz-justify-content: flex-start;
justify-content: flex-start;
position: relative;
}
.order-form .left-column {
overflow: hidden;
display: -webkit-box;
display: -webkit-flex;
display: -moz-flex;
display: -ms-flexbox;
display: flex;
-webkit-flex-basis: 0;
-moz-flex-basis: 0;
flex-basis: 0;
-webkit-order: 1;
-moz-order: 1;
order: 1; }
#media (min-width: 992px) {
.order-form .left-column {
margin-right: 5%;
display: -webkit-box;
display: -webkit-flex;
display: -moz-flex;
display: -ms-flexbox;
display: flex;
-webkit-flex-basis: 100px;
-moz-flex-basis: 100px;
flex-basis: 100px;
}
}