Precision gap: self-detaching event handler (unsubscribes itself in its own body) flagged as unbounded #224

Found by: the issue #201 oracle sweep, docs/notes/oracle-sweep-2026-07-10.md; catalogued as docs/notes/field-notes-patterns.md entry 18.

The pattern

DropDownContent.Closed += DropDownContent_Closed;   // <- flagged: no visible -= near the +=

void DropDownContent_Closed(object sender, EventArgs e) {
    ((Popup)sender).Closed -= DropDownContent_Closed;   // removes itself, first firing
    this.IsDropDownContentOpen = false;
}

The handler removes itself from the event the first time it runs — a common one-shot idiom ("do this once, then stop listening") that needs no external detach call. By the time the event could fire a second time, the subscription is already gone.

Own.NET's -= search is scoped to the subscribe call's enclosing method (or a class-level scan for a field-based handler); it doesn't look inside the handler's own body for a matching self-removal.

Evidence

• AvalonEdit ICSharpCode.AvalonEdit/Search/DropDownButton.cs:78-86

Suggested direction (not prescriptive)

When resolving whether a subscribed handler has a matching -=, also check the handler method's own body for a -= against the same event/delegate expression (typically via the sender parameter cast back to the source type) — a handler that detaches itself is bounded by construction.

Scope

No analyzer code changed as part of the sweep — this issue tracks the fix as its own unit of work.