diff --git a/plans/hs-conformance-scoreboard.md b/plans/hs-conformance-scoreboard.md
index 9a9e62d2..0fc366a3 100644
--- a/plans/hs-conformance-scoreboard.md
+++ b/plans/hs-conformance-scoreboard.md
@@ -4,10 +4,10 @@ Live tally for `plans/hs-conformance-to-100.md`. Update after every cluster comm
 
 ```
 Baseline:   1213/1496 (81.1%)
-Merged:     1253/1496 (83.8%)   delta +40
-Worktree:   2 in flight (clusters 20, 27)
+Merged:     1256/1496 (84.0%)   delta +43
+Worktree:   1 in flight (cluster 20)
 Target:     1496/1496 (100.0%)
-Remaining:  ~243 tests
+Remaining:  ~240 tests
 ```
 
 ## Cluster ledger
@@ -52,7 +52,7 @@ Remaining:  ~243 tests
 | # | Cluster | Status | Δ | Commit |
 |---|---------|--------|---|--------|
 | 26 | resize observer mock + `on resize` | done | +3 | 304a52d2 |
-| 27 | intersection observer mock + `on intersection` | pending | (+3 est) | — |
+| 27 | intersection observer mock + `on intersection` | done | +3 | 0c31dd27 |
 | 28 | `ask`/`answer` + prompt/confirm mock | pending | (+4 est) | — |
 | 29 | `hyperscript:before:init` / `:after:init` / `:parse-error` | pending | (+4–6 est) | — |
 | 30 | `logAll` config | done | +1 | 64bcefff |
@@ -87,7 +87,7 @@ Defer until A–D drain. Estimated ~25 recoverable tests.
 |--------|-----:|--------:|--------:|--------:|--------:|------------:|------:|
 | A      | 12   | 4       | 0       | 0       | 1       | —           | 17    |
 | B      | 3    | 0       | 0       | 3       | 1       | —           | 7     |
-| C      | 2    | 0       | 0       | 3       | 0       | —           | 5     |
+| C      | 3    | 0       | 0       | 2       | 0       | —           | 5     |
 | D      | 0    | 0       | 0       | 5       | 0       | —           | 5     |
 | E      | 0    | 0       | 0       | 0       | 0       | 5           | 5     |
 | F      | —    | —       | —       | ~10     | —       | —           | ~10   |
diff --git a/plans/hs-conformance-to-100.md b/plans/hs-conformance-to-100.md
index 17a15273..0c10c074 100644
--- a/plans/hs-conformance-to-100.md
+++ b/plans/hs-conformance-to-100.md
@@ -103,7 +103,7 @@ Orchestrator cherry-picks worktree commits onto `architecture` one at a time; re
 
 26. **[done (+3)] resize observer mock + `on resize`** — 3 tests. Add a minimal `ResizeObserver` mock to `hs-run-filtered.js`, plus parse/compile `on resize`. Expected: +3.
 
-27. **[pending] intersection observer mock + `on intersection`** — 3 tests. Mock `IntersectionObserver`; compile `on intersection` with margin/threshold modifiers. Expected: +3.
+27. **[done (+3)] intersection observer mock + `on intersection`** — 3 tests. Mock `IntersectionObserver`; compile `on intersection` with margin/threshold modifiers. Expected: +3.
 
 28. **[pending] `ask`/`answer` + prompt/confirm mock** — `askAnswer` 4 tests. **Requires test-name-keyed mock**: first test wants `confirm → true`, second `confirm → false`, third `prompt → "Alice"`, fourth `prompt → null`. Keyed via `_current-test-name` in the runner. Expected: +4.
 
@@ -175,6 +175,9 @@ Many tests are `SKIP (untranslated)` because `tests/playwright/generate-sx-tests
 
 (Reverse chronological — newest at top.)
 
+### 2026-04-24 — cluster 27 intersection observer (worktree re-apply)
+- **0c31dd27** — `HS: intersection observer mock + on intersection (+3 tests)`. Worktree agent `ad6e17cb…` produced the logical change but its commit's reformatted compiler/runtime bodies conflicted with cluster 26's landing of resize-observer state in `tests/hs-run-filtered.js` and `runtime.sx`; straight cherry-pick hit markers on `.sx` files that can't be Edit'd. Reapplied surgically via sx-tree: (a) parser `parse-on-feat` now collects `having margin X threshold Y` clauses between `from` and body, packaging them in `:having {"margin" M "threshold" T}`; (b) compiler `scan-on` threads a new `having-info` param and, when event-name is `"intersection"`, wraps `(hs-on target "intersection" fn)` with `(do on-call (hs-on-intersection-attach! target margin threshold))`; (c) runtime adds `hs-on-intersection-attach!` which constructs an `IntersectionObserver` with `{rootMargin, threshold}` opts and a callback that dispatches an `intersection` DOM event carrying `{intersecting, entry}` detail; (d) runner adds `HsIntersectionObserver` stub that fires the callback synchronously on `observe()` with `isIntersecting=true`. Suite hs-upstream-on: 33/70 → 36/70. Smoke 0-195 unchanged at 165.
+
 ### 2026-04-24 — cluster 26 resize observer (worktree agent cherry-pick)
 - **304a52d2** (from worktree `worktree-agent-a8983e935d0d7a870` / `aea5f7d2`) — `HS: resize observer mock + on resize (+3 tests)`. `tests/hs-run-filtered.js`: `El.style` becomes a `Proxy` that fires a synthetic `resize` DOM event (detail carries `{width, height}` parsed from the current inline style) whenever `width`/`height` is written via direct property assignment OR `setProperty`. `ResizeObserver` no-op stub replaced by `HsResizeObserver` maintaining a per-element callback registry in `globalThis.__hsResizeRegistry`; added `ResizeObserverEntry` stub. `on resize` needs no parser/compiler work — `parse-compound-event-name` already accepts it and `hs-on` binds via `dom-listen`. Generator: new pattern translates `(page.)?evaluate(() => { document.(getElementById|querySelector)(...).style.PROP = 'VAL'; })` into `(host-set! (host-get TARGET "style") "PROP" "VAL")`. Suite hs-upstream-resize: 0/3 → 3/3. Smoke 0-195 unchanged at 165/195.