Allow top layer elements to be nested within popovers

Category: DOM
Type: Chromium catches up
Status: Enabled by default (Chrome 125)
Intent stage: None

Summary

The prior behavior caused issues if a top layer element (dialog, fullscreen, etc.) was DOM-nested within a popover. The inner element, when promoted to the top layer, would cause the outer popover to be hidden, which also (via CSS display:none) hid the inner element. That, in turn, caused issues at least for dialogs, which inert the entire page. This change makes top layer elements "nestable" within popovers.

Motivation

See https://github.com/whatwg/html/issues/9998

Standards & signals

Specification: https://github.com/whatwg/html/pull/10116
Firefox: Shipped/Shipping — https://phabricator.services.mozilla.com/D200686
Safari: Positive — Informal support at WHATNOT meeting: https://github.com/whatwg/html/issues/10052#issuecomment-1911240732
Web developers: Positive — Issue raised by developers
Tracking bug: https://crbug.com/1520938

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