I think the issues comes from that when the scrollbar disappear, style="margin-right: 17px; overflow: hidden; is applied on body to compense the scrollbar width.
But .d-header being in fixed position, it doesn’t “inherits” the html margin-right.
edit:
It seems that the html margin (equals the scrollbar width) is calculated via javascript through jquery.magnific-popup.min.js.
I’m not sure there is any quick and clean solution to fix this. 