It turns out there’s something *very* similar in Microsoft Edge’s Devtools, though it appears to use floating panes instead of stacked boxes (maybe? the screenshots are unclear and other screenshots I’ve seen appear to be pane-based).

Regardless, it might be of use — particularly since they **also** added a `z-index` stacking context visualizer! https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/3d-view/