test/split-pane-auto-collapse.browser.test.ts covers "Pane B's session ends"
in a real Chromium, but that suite is excluded from the npm test CI gate.
The "Pane A's session ends, Pane B gets promoted" branch had no coverage
anywhere, and it is the one branch whose correctness depends on exact
ordering: _splitSessionId must be captured BEFORE closeSplitPane() runs
(which nulls it) or the promoted session id is lost. Loads terminal-split.js
via `vm` against a minimal fake CodemanApp (same technique as
test/session-close-fallback.test.ts), and pins all three branches (Pane A
ends, Pane B ends, unrelated session ends) plus that the original
_onSessionDeleted always still fires.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
