Issue 23513 — before runtime transcript
Commit: ff525478e2 (origin/main at triage start)
Environment: fs-14 Linux x86_64; vp run dev in /home/ubuntu/wt/issue-23513;
web port 8564, server port 16604; isolated copied .t3 state.
Browser: product-native server preview, headless Chrome, 1280x800.

Fixture served on 127.0.0.1:18913:
<button>A</button><script>
for (const type of ["beforeunload", "pagehide", "visibilitychange"])
  addEventListener(type, () => navigator.sendBeacon(`/beacon?${type}`));
</script>

Attempt 1: no user activation, current-main real app
Opened through the paired web app's previewEnvironment.open command with
runtime:'server' and url:'http://127.0.0.1:18913'.
09:28:27 GET / 200
09:28:40 previewEnvironment.close succeeded
09:28:40 POST /beacon?visibilitychange 204
09:28:40 POST /beacon?pagehide 204
No beforeunload beacon.

Attempt 2: literal reporter tool sequence, installed host runtime (separate
from the isolated current-main dev server; corroborating evidence only)
09:29:31 preview_open http://127.0.0.1:18913/?attempt=tool; GET 200
preview_snapshot identified button A; preview_click A succeeded
09:29:54 t3_preview_close succeeded
09:29:54 POST /beacon?pagehide 204
09:29:54 POST /beacon?visibilitychange 204
No beforeunload beacon.

Attempt 3: with user activation, current-main real app
09:29:15 opened through the paired web app previewEnvironment.open command.
Attached its production PreviewStreamClient to the server preview stream.
Control returned canOperate:true, controller:'you'. Sent mouse down/up
at button A (20,18); both sends returned true.
09:30:40 previewEnvironment.close succeeded
09:30:40 POST /beacon?visibilitychange 204
09:30:40 POST /beacon?pagehide 204
No beforeunload beacon.

Result: only beforeunload omission reproduced. Current main does deliver
pagehide and visibilitychange, contrary to the issue's broader claim.