You gave me the CSS-code below to make the popup top-bar disappear:
Code
.pswp__top-bar,
.pswp__caption {
background-color: rgba(0,0,0,0);
}
Code
<a href="{{path}}emptiness_tarmac.jpg" data-popup data-width=2048 data-height=1365><img class=x3-style-frame src="{{path}}emptiness_tarmac.jpg" /></a>
It is even so that after clicking on the above image all other regular popups (i.e. when clicking on an image in a gallery) are not invisible anymore, but grey. Only after clearing my cache, these last images have their invisible topbars again.