EditorRegistry.close() guarded its active-changed emit on `_activeId !=
null`, so closing the LAST buffer (active clears to null) emitted only
editor.closed — never the active-changed(id:null) the editor extension
listens for to call closeEditor(). editorOpen stayed true and the top
split sat orphaned over the Claude pane. Always emit active-changed when
the active buffer is removed, including the cleared-to-null case; the
slot renderer already collapses correctly once editorOpen flips false.
The existing extension test fabricated the null active-changed event, so
it passed despite the registry never emitting it — that gap is why the
bug shipped. Add a registry test that drives the real close() path, plus
a slot_host widget test asserting the split (drag handle) drops out and
the primary pane fills the column.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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