[ONEGAYI]

Problem

VS Code never reloads a dirty text document when its file changes on disk — no reload, no event. The markdown editor applies webview input to the document via applyEdit and only writes the file on save, so the document is dirty almost constantly. As a result:

• external writers (formatters, AI agents, other editors) silently vanish from an open editor view, and
• the next manual save silently clobbers their changes with the stale buffer.

This is very visible when an agent edits files while the user has them open in the WYSIWYG markdown editor.

Fix

The Handler already owns a FileSystemWatcher whose fileChange events were never consumed by anyone — this PR consumes them for the markdown editor:

• Pure decision core (src/service/markdown/externalChangeGuard.ts): strict UTF-8 normalization (non-UTF-8 files are left alone — VS Code may decode documents with other encodings, and adopting a mojibake interpretation would corrupt them) and shouldAskAboutDiskChange: ask iff the document is dirty, the disk text differs from every current/recent buffer form (echo suppression), and the user has not already declined that exact text.
• Prompt: "Load disk version" / "Keep my edits" (ext.markdown.externalFileChange, localized in all 10 package.nls files).
  • Load re-reads the disk right before applying (never a stale snapshot), clears the pending sync, applyEdit + document.save() + pushes the content to the webview. Same contract as VS Code's own conflict flow: local unsaved edits are discarded.
  • Keep (and dismissing the prompt via Esc) records the declined text — asked once per distinct disk content, so streaming writers don't spam.
• Save guard: workspace.onWillSaveTextDocument + event.waitUntil parks saves of the document while a decision is pending, so autosave or a reflexive Ctrl+S cannot silently overwrite the change the prompt is asking about.
• Echo suppression for focus-outside saves: recent pre-flush document snapshots (bounded ring) are treated as buffer forms.
• A failed save after loading warns instead of failing silently (ext.markdown.externalSaveFailed).
• Unit tests for the decision core: npm run test:unit (node:test; 10 cases incl. GBK/UTF-16 rejection, echo orderings, emptied disk). Note: requires Node ≥ 22.6 for native TS type stripping.

Known limitations (deliberately out of scope)

• Same file open in two panels → duplicate prompts, and a Load in one panel overwrites the other panel's unsaved edits (pre-existing shared-document model, made visible by this feature)
• Watcher blind spots: non-workspace virtual schemes have no watcher; file names containing glob metacharacters (report[1].md) silently fail to match; deletes/renames produce no prompt
• Millisecond read-modify-write races remain around Load (no atomic file API exists)
• If the prompt is relegated to the notification center and ignored, saves of that document wait until it is answered

Validation

• npm run test:unit — 10/10 pass
• npm run build — pass
• npx eslint on changed files — clean

Design reference: ONEGAYI/vscode-markdown-editor-hardened (external-change-watcher), adapted to this extension's handler/sync model with two review rounds of hardening (UTF-8 strictness, prompt serialization, save-guard deadlock avoidance, echo rings).

Note: this PR touches the import block of markdownEditorProvider.ts and adds the same two config lines (test script / tsconfig exclude) as #610 — expect a trivial merge conflict if that lands first.

[ONEGAYI]

Closing due to inactivity — no review feedback after ~3 weeks. The commits remain available via refs/pull/611/head in this PR if anyone wants to revisit. Thanks!