Would be useful to be able to add XYZ tiles as a layer on top of a basemap, rather than replacing the basemap

[mapninja]

I'm currently working with https://iiif.io and https://allmaps.org to create infrastructure for native iiif (international image interoperability framework) image georeferencing.

I was just working on workshop materials for taking allmaps georeferenced maps and heads up digitizing their features, in geojson.io, but the fact the the only xyz tile option is to replace the basemap is not optimal for this use case.

It would be great to have the option to add the xyz tiles as a layer, rather than a basemap, to retain the basemap.

Below is a screenshot, showing the allmaps xyz tiles, floating on a blank canvas.