Symptom
The site header or the navigation stays on top of the lightbox when a gallery image is open.
Cause
The theme’s header has a z-index higher than the lightbox’s 1500.
Fix
Raise the lightbox above it. Open Appearance → Customize → Additional CSS and add:
body .vp-pswp,
body .vp-fancybox {
--vp-popup__z-index: 9000;
}Raise the value further if the header still wins.