Resolves #14928

Related: PR #14920 (the discovery + shipped call-site workarounds) · #14909 (the lane that surfaced it)

core.Observable's object-form addListener AND removeListener deleted the reserved keys (scope, plus once/delay/order in add) off the CALLER's object while parsing. The natural symmetric idiom — one listeners object through newSubject.on(listeners); oldSubject.un(listeners) — therefore silently never unbound: on() consumed scope, un() matched nothing, and the old subject kept driving its consumer. This bit two framework call sites this week (grid.Container.afterSetStore latently since introduction; grid.VerticalScrollbar the moment store replacement became reachable), and Observable ITSELF already carried an internal workaround: updateConfiguredListeners passes "FRESH per-event slices" with a comment naming exactly this corruption. The fix moves that discipline to the API boundary — both object-form branches parse a shallow COPY (name = {...name}) — so no caller ever needs to know.

Behavior is otherwise identical: same parse, same stored listener shapes, same matching. The caller-object mutation was never a documented contract, and no caller reads back the consumed keys (the internal workaround exists precisely because mutation was a hazard, not a feature). The per-call copies PR #14920 shipped at the two grid sites stay — harmless, and their comments document the historical trap for archaeology.

Evidence: L1 (unit — the defect and fix are single-class semantics; the full core/component/grid/state trees are the behavior-preservation guard). No residuals.

Deltas from ticket

• Ticket's implementation-time check ("other Observable surfaces sharing the destructive parse") ran: grep 'delete ' over the class shows exactly the two fixed branches plus the afterSetListeners internal workaround — no third site.
• One pre-existing 3-line alignment drift in afterSetListeners fixed as its own commit (whole-file CI gate; the fixer's diff is exactly those 3 lines).

Test Evidence

• New spec test/playwright/unit/core/ObservableListenerObjects.spec.mjs (4 tests): on() non-mutation (all reserved keys survive) · un() non-mutation · the shared-object SWAP idiom (new.on(listeners); old.un(listeners) → old actually unbinds, only 'new' fires) · the same-subject on/un round-trip over one shared object.
• Teeth-check (stash-run-unstash at this head): all except the on()-mutation case FAIL without the fix — including the same-subject round-trip (even subject.on(L); subject.un(L) never unbound pre-fix).
• Behavior-preservation guard: full core + component + grid + state unit trees, no-Chroma config --workers=1: 186 passed / 0 failed at the fix head.
• node --check + check-block-alignment + check-ticket-archaeology green on both touched files.

Post-Merge Validation

• PR #14920's call-site copies ({...listeners} in grid.Container / VerticalScrollbar) remain correct-and-redundant — optional polish may simplify them later; nothing depends on it.
• updateConfiguredListeners' fresh-slice workaround likewise stays valid; its comment now describes a hazard the boundary no longer has (follow-up polish candidate, not behavior).

Commits

• 4f3264043 — the boundary copies in addListener/removeListener + the 4-test spec.
• e895cd1dc — pre-existing afterSetListeners alignment drift (3 lines, whole-file gate).

Authored by Vega (Claude Fable 5, Claude Code). Session d2fbbdb4-404b-47e1-bbb3-1b9e0330894b.