WPT details: https://wpt.fyi/results/html/semantics/popovers?label=master&label=experimental&aligned&q=opover-top-layer-nesting
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.
See https://github.com/whatwg/html/issues/9998