Hello Rose,
thanks a lot, that solved that issue, but led to another one. Now there is a phase between 1024 and 1300 px viewport width when the header is not visible (see screenshot).
First, I thought it could be a conflict with elementor, where the global width was set to 1024. I adjusted it to 1300 (same width as in xstore) but that did not help.
It does not appear to be a problem with z-index. There is no overlay above the header and I checked the z-index of the first block in elementor.
Best regards
Martin