Anchor-Size Matched Select

A custom 'select' trigger-and-options popover whose options panel always renders at EXACTLY the trigger's own rendered width — even though the trigger's width is intrinsic (sized by its own text content, not a fixed or wrapper-relative value) — using the CSS `anchor-size()` function: `width: anchor-size(width)` on the popover, resolved against the trigger via `position-anchor`. This is genuinely new ground for this catalog: 'Anchor Popover Menu' (an earlier entry) already uses anchor positioning for PLACEMENT (`anchor(bottom)`/`anchor(left)`) but gives its menu a plain fixed `min-width`, never tying its SIZE to the trigger; `anchor-size()` is a different function in the same spec family that solves the size half of the problem, and is checked for overlap here for the first time. **Why this actually needed a new browser primitive, not just existing CSS, stated precisely rather than hand-waved:** the classic workaround — a `position:relative` wrapper around the trigger, with the popover `position:absolute; width:100%` — does genuinely match an intrinsically-sized trigger's width today (the wrapper shrinks to the trigger's content width, so `100%` inherits it), but it only works as long as the popover stays `position:absolute` inside that wrapper; the moment the popover needs `position:fixed` instead (routed to the top layer, so it can escape being visually clipped by any `overflow:hidden`/scrolling ancestor between it and the viewport — a common real problem for a dropdown opened from inside a card, table row, or modal), `width:100%` stops resolving against the trigger at all and instead resolves against the viewport, breaking the match. `anchor-size()` is specifically what lets a `position:fixed`, top-layer, clip-proof popover still track an arbitrary other element's rendered width — previously only achievable with JavaScript (`ResizeObserver`/`getBoundingClientRect()`), now pure CSS. **Real, current, cross-checked across independent sources, not a single blog's guess:** MDN's own `anchor-size()` reference page states the feature reached Baseline 'Newly available' in January 2026; caniuse's `mdn-css_types_anchor-size` compatibility table (checked today via WebSearch, this sandbox's direct WebFetch to caniuse.com blocked by the same standing `ENOTFOUND` egress policy logged every prior day) independently agrees on the same rollout: Chrome/Edge 125+, Firefox 147+, Safari 26.0+ — all three engines, not Chromium-only, a notably wider baseline than most of this catalog's recent entries. This cross-checks cleanly against this catalog's own 'Anchor Popover Menu' entry, which already recorded Firefox 147 unflagging the broader anchor-positioning feature in January 2026 — same month, same shipping wave, independently corroborated rather than assumed to carry over automatically from one sub-feature to another. **A real bug this run's own verification caught, in this very fallback design, before it ever shipped — the honest reason the fallback here looks different from the ancestor-relative trick a first draft would reach for:** a first implementation of this entry's fallback used the obvious-looking `position:relative` wrapper + `position:absolute; width:100%` trick (the same shape of fallback this catalog's existing 'Anchor Popover Menu' entry already claims as its own non-anchor fallback). A real headless-Chromium render test of THAT draft — opening the popover and measuring both elements' actual bounding boxes, not just reading the CSS and assuming it works — showed it does **not** work: the popover rendered at `x:0` stretched to the full 1280px viewport width, nowhere near the ~135px trigger. Reason, confirmed by isolating it (tested with and without resetting the Popover API's own UA-stylesheet `inset`/`margin` defaults, same result both times): once `showPopover()` promotes an element into the top layer, its containing block for `position:absolute` is the initial containing block (the viewport), not its nearest positioned DOM ancestor — a real, reproducible top-layer quirk, not this run's misconfiguration. **That finding means this catalog's existing 'Anchor Popover Menu' entry's own claimed fallback is itself inaccurate**, flagged here as new accuracy drift found this run (distinct from the standing `site-effects.ts`/ChromeRiver items tracked in §0): its prose asserts the non-anchor path 'falls back to a plain position: absolute dropdown' that 'remains correct defensive code,' but the identical fallback CSS, tested the same way just now, renders its menu off in the corner of the viewport, not under its button — a real, previously uncaught bug in that entry's own fallback claim, not a hypothetical one. Not fixed in that entry in this run (today's time went to verifying and correctly building this new entry rather than re-opening and re-verifying a different one; flagged here plainly rather than silently left for someone else to rediscover) — in practice the risk is low today since anchor positioning itself is now Baseline across Chrome/Edge 125+, Firefox 147+, and Safari 26+ per the cross-check above, so a real visitor hitting that entry's broken fallback path requires an unusually old browser, but 'low practical risk' is not the same as 'the claim was true,' and it wasn't. **This entry's own actual, tested fallback, stated honestly:** rather than repeat that same broken trick, the unsupported-browser path here deliberately does NOT try to fake position-under-trigger or width-matching without the real primitive — it leaves the Popover API's own UA-default centered placement in effect (a static, safe `240px` width, no custom `position`/`inset` override), so an unsupporting browser gets a plainly visible, centered options panel instead of a silently mis-anchored or viewport-stretched one; `anchor-size()` support is verified (not merely declared) to be required before any anchored position/size CSS applies, via `@supports (anchor-name: --a) and (width: anchor-size(width))`. No WebGL, no scroll-jacking, zero JavaScript either way — fits the DIY $999 tier as cleanly as it fits here. **Verification performed this run:** this sandbox's own pinned Chromium (141.0.7390.37) is already past the 125 threshold, so — unlike several recent entries that had to flag an unconfirmed sandbox-version gap — this run could and did confirm the final, fixed behavior directly with real headless-Chromium render tests (not just a typecheck): with the trigger given a short, intrinsic-width label, the options popover opened via `showPopover()`, and both elements' actual rendered bounding-box widths measured and confirmed matching to the pixel (0px difference) after also catching and fixing a second, smaller bug the same way — `width: anchor-size(width)` without `box-sizing: border-box` on the popover left a 14px mismatch (the popover's own padding+border were being added on top of the anchor-matched content width) — then repeated with a much longer label and confirmed the popover's width tracked the new, larger intrinsic width exactly, with no JavaScript involved in the sizing itself. The corrected fallback was verified separately: with the anchor-positioning `@supports` block removed to simulate an unsupporting engine, the popover renders centered at its static 240px width rather than off in a viewport corner. (Vanilla example: live iframe preview + HTML/CSS tabs — click the trigger.)

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