Select active GPT responsive slots

[ChristianPavilonis]

Summary

• Resolve stable GPT slot prefixes to the unique active responsive sibling instead of the first matching DOM element.
• Preserve exact-ID and unique lazy-slot behavior while failing closed when multiple prefix matches have no unique visible, laid-out candidate.
• Keep the edge bootstrap and runtime implementation aligned with shared regression coverage.

This PR is stacked on #966, which already preserves GPT's display(string | Element | Slot) contract in the permanent slot-handoff implementation. It does not depend on #974.

Changes

File	Change
crates/trusted-server-core/src/integrations/gpt_bootstrap.js	Select a unique active responsive prefix match before defining or reusing a GPT slot.
crates/trusted-server-js/lib/src/integrations/gpt/index.ts	Mirror responsive slot resolution in the runtime integration and fail closed on ambiguity.
crates/trusted-server-js/lib/test/integrations/gpt/ad_init.test.ts	Cover runtime/bootstrap parity for element and container layout, hidden siblings, ambiguity, empty IDs, and publisher-owned slots.
crates/trusted-server-core/src/integrations/gpt.rs	Update the injected-bootstrap source assertion for the shared resolver.

Closes

Closes #977

Test plan

• cargo test-fastly && cargo test-axum && cargo test-cloudflare && cargo test-spin
• cargo test --manifest-path crates/trusted-server-integration-tests/Cargo.toml --test parity
• cargo clippy-fastly && cargo clippy-axum && cargo clippy-cloudflare
• cargo clippy-cloudflare-wasm && cargo clippy-spin-native && cargo clippy-spin-wasm
• cargo fmt --all -- --check
• JS tests: cd crates/trusted-server-js/lib && npx vitest run (430 passed)
• JS lint and format: npm run lint && npm run format
• JS production build: npm run build
• Docs format: cd docs && npm run format
• Fastly WASM release build: cargo build --package trusted-server-adapter-fastly --release --target wasm32-wasip1
• Manual testing via fastly compute serve

Checklist

• Changes follow CLAUDE.md conventions
• No unwrap() in production code — use expect("should ...")
• Uses log macros (not println!)
• New code has tests
• No secrets or credentials committed

[prk-Jr]

Summary

Replaces first-match-in-document-order prefix resolution with "unique active prefix match", failing closed on ambiguity, and mirrors it across the edge bootstrap and the bundle with a shared test table. The direction is right and the parity coverage is good. One blocking issue: the resolver requires geometry before it will pick a sibling, so the single visible responsive sibling is still skipped when it reserves no space — and unlike before this PR, nothing is defined or requested at all in that case.

Blocking

🔧 wrench

• Geometry gate drops the one visible responsive sibling when it reserves no space: slotElementHasLayout requires width > 0 && height > 0 on the element or its -container. An empty ad div awaiting its creative commonly has height 0, and its empty wrapper does too, so the active sibling fails the check and activeMatches.length === 0 → null → the slot is skipped entirely (no defineSlot, no ad request). Before this PR the first prefix match was defined and requested. Verified against both implementations with a throwaway probe (three display:none dupes plus one visible sibling, default 0×0 rects):

  GPT slot prefix did not resolve to one active element { divId: 'ad-probe-', prefixMatchCount: 4, activeMatchCount: 0 }
  → defineSlot not called, divToSlotId {}
  

  Adding a visibility-only tier before the geometry tier resolves the not-size-a/b/c/d pattern this PR targets without requiring reserved height. Details inline on gpt_bootstrap.js:121 and gpt/index.ts:85.

Non-blocking

🤔 thinking

• Forced synchronous layout inside the SPA MutationObserver (crates/trusted-server-js/lib/src/integrations/gpt/index.ts:897): waitForSlotElements calls allPresent() on every mutation batch, and findSlotElementByDivId now runs getComputedStyle + getBoundingClientRect per prefix candidate whenever a prefix has more than one match. During framework hydration that is a forced reflow per batch. The presence check only needs existence, not activeness — consider a geometry-free probe there (or a resolver flag that stops at the prefix-match tier).
• Ambiguity warn never fires from the edge bootstrap — see inline on gpt_bootstrap.js:129.

🌱 seedling

• Resolver logic now exists twice (edge bootstrap JS and bundle TS), kept aligned by a substring assertion in gpt.rs. Worth a follow-up to generate the bootstrap from one source — see inline on gpt.rs:1216.

⛏ nitpick

• Test table covers ambiguous, none-active, hidden-sibling and empty-div_id cases, but not "exactly one visible sibling, nothing laid out" — the case that motivates the blocking finding — and nothing asserts the warn payload (prefixMatchCount / activeMatchCount), which is the only diagnostic for the fail-closed path.

👍 praise

• Empty-div_id guard and the runtime/bootstrap parity table — see inline on gpt/index.ts:74 and ad_init.test.ts:1298.

CI Status

Only the Integration Tests workflow ran on 340d1efb4 (4 checks, all pass). format.yml and test.yml trigger on pull_request: branches: [main, "feature/**"], so with this PR based on fix/duplicate-gpt-slots the fmt / clippy / Rust test / JS test gates did not run on CI. Verified locally on the head commit instead:

• fmt: NOT RUN on CI
• clippy: NOT RUN on CI
• rust tests: PASS locally (cargo test -p trusted-server-core --target aarch64-apple-darwin gpt — 31 passed)
• js tests: PASS locally (npx vitest run — 30 files, 430 passed)
• integration tests: PASS on CI