A rendering issue seems to have been introduced in MEC version 220.127.116.11. Difficult to describe in words so here's a picture from a online game called Tribal Wars 2.
The right screenshot from existing Windows Edge shows the correct rendering of the floating box, note the two icons, one a building the other crossed swords. Underneath the building tab icon, the various building icons stack correctly. On the left is MEC 18.104.22.168, the building icons aren't stacking correctly, the icons appear to be behind rather than below the main tab icon. They are also obscure in front of the second tab icon of cross-swords.
Reported via feedback but not as obvious as I couldn't upload a side-by-side example