Anchored popover (combobox / menu)
Use CSS anchor positioning instead of JS measurement:
anchor-name on the field; on a popover="manual" element set
position-anchor, top: anchor(bottom), width: anchor-size(width), and
position-try-fallbacks: flip-block so it opens downward and flips up near the
viewport bottom. It lives in the top layer (no z-index juggling). Chromium
125+; add an @supports fallback if Firefox/Safari matter.
Two scars to carry (gf-79, gf-46): a CLOSED .menu popover must get
display: none — hiding by opacity alone leaves every hidden menu in the
tab order. And out-of-flow anchor positioning LOSES to morphs: the moment
the server removes or replaces the anchor node, everything anchored to it
jumps or orphans. Anchor to stable, server-owned nodes only; anything that
chains or stacks (toasts) stays IN-FLOW with interpolate-size collapse —
degrade is a snap, never a break. An overlay that traces an element pins
to THE TRACED ELEMENT itself, never its wrapper (hold-confirm's inset:0
mis-scale).