I believe I have found the bug. It is reproducible without any css changes. The code in headerBuilder.js runs once and whatever the main header height is at that time is stored. If the user then resizes the desktop browser window to a responsive threshold for mobile, the main header height is smaller but the value used in headerBuilder.js is not recalculated. The fix would be to recalculate the headerHeight and thus reset paddingTop whenever the mainHeader height changes. This fix should be made in the Woodmart theme directly and is not just a patch for my site.

Let me know if you agree or have any other questions for me. Thank you.