The document editor stores the AI-edit diff state (_diffModeActive,
_diffOldContent, _diffNewContent, _diffChunks) as a module-global
singleton bound to whatever document was active when the diff opened,
and every document shares one #doc-editor-textarea. When the active
document is switched while an unapproved diff is open, the stale diff
must be discarded first or a later exitDiffMode (tab switch /
Accept-Reject-All) flushes the old document's content into the new
active document and overwrites it (issue #2467).
Guard both paths that switch the active document for an AI update,
while activeDocId still points at the previously-active doc:
- handleDocUpdate(): a doc_update targeting a different document.
- streamDocOpen(): the AI streaming a NEW document — this runs first on
that path, so a guard only in handleDocUpdate would fire too late and
still overwrite the streamed document.
Both reuse the exact `if (_diffModeActive) exitDiffMode(true);` guard
switchToDoc() and enterDiffMode() already use.
Fixes#2467
nubs