I believe a fix I just pushed should cover this:
https://github.com/discourse/discourse/commit/70acfa47d7e36a2a2798d9c2042c0aadd1130ea0
I had updated the way the layout of the menu works a bit (added flexbox, which helps accommodate font scaling)… and it seems like Safari was including the width of the expanded hamburger menu where it shouldn’t have been. You can try this fix out on your sites by adding this CSS to your customization:
.hamburger-panel {
width: 0;
}