Thank you for reporting this behavior and for testing it on macOS. The issue was related to the Sticky Header configuration. You had the Sticky Offset set to 0, which behaves normally in most browsers. However, in Safari the page position can go slightly beyond 0 (for example -10) when scrolling upward using trackpad gestures. Because the header has an animation between sticky and non-sticky states, this caused a brief flicker that looked like a soft refresh of the header.
I have now adjusted the offset to a negative value, which aligns the transitions correctly and prevents the flickering in Safari. The issue should be resolved on your client’s MacBook.
If you notice anything else during testing, feel free to let me know.