Not an overlay, but side by side views are shown, when scrolling down enough, in the French article linked by @thinkingremote : https://www.inrap.fr/la-plus-anc

Not an overlay, but side by side views are shown, when scrolling down enough, in the French article linked by @thinkingremote : https://www.inrap.fr/la-plus-ancienne-carte-d-europe-15574# https://www.inrap.fr/la-plus-ancienne-carte-d-europe-15574#