GitHub-flavored markdown editor as a VS Code custom editor for .md / .markdown files. Runs in VS Code Desktop AND vscode.dev / github.dev.

Features

Default editor for .md files (3-pane: source / preview / outline)
Mermaid pan and zoom (matching mermaid.live UX)
GitHub-flavored alerts, tables, fenced code highlight (via starry-night)
Per-pane light / dark themes (cycle independently)
Resizable splitters; widths cached across sessions via globalState
Tab swap stays snappy — webview state preserved (retainContextWhenHidden)

Build (local)

From the repo root, one command does the clean-build-and-package:

npm run build:ext                    # -> vscode/gh-md-editor-X.Y.Z.vsix

Or drive the whole release — version bump, build, package, safety gates, commit, push:

./scripts/ship.sh release ext 0.2.8
./scripts/ship.sh publish ext        # separately, once you have a Marketplace token

The underlying steps, if you'd rather run them by hand:

cd vscode
npm install
npm run build         # esbuild dual-target + vite webview
npm run package       # produces gh-md-editor-X.Y.Z.vsix

The parent project (../) must already have node_modules populated (npm install once at the top level). The webview Vite build aliases @gmd → ../src so Editor / Preview / Outline / Splitter Svelte components are shared with the web app build.

Install

VS Code → Extensions → ... menu → Install from VSIX... → select gh-md-editor-X.Y.Z.vsix
Open any .md file; it opens in the GH MD Editor by default.
To temporarily use the default text editor: right-click file → Reopen With... → Text Editor.

Architecture

src/extension.ts — extension host. Registers CustomTextEditorProvider, bridges webview ↔ TextDocument via postMessage.
webview/main.ts — webview entry. Waits for init from host, then mounts the Svelte app.
webview/index.html — Vite dev template (production HTML built at runtime by extension host with CSP nonces).
vite.config.ts — webview bundler; aliases @gmd → parent project's src/ so Editor / Preview / Outline are shared verbatim with the web app.
esbuild.config.mjs — extension host bundler (dual-target: Node desktop + webworker for web).

Message Protocol

Direction	Type	Payload	Purpose
webview → host	`ready`	—	Webview booted, ready for init
host → webview	`init`	`{ content, widths, uri, languageId }`	Initial document content + cached widths
webview → host	`editorChange`	`{ text }`	Full document replacement (Phase 2: structured `changes[]` for proper undo granularity)
webview → host	`widthsChange`	`{ splitPct, outlineSplitterPct }`	Persist splitter widths to globalState
host → webview	`externalUpdate`	`{ text }`	External edit (other extension, multi-cursor scenario) — re-dispatch to CM6

Known limitations (Phase 1)

Edit flow uses full-document replace per change; breaks VS Code's per-keystroke undo granularity. Cmd+Z still works but bundles many keystrokes at once.
Mermaid requires 'unsafe-eval' in webview CSP (uses new Function() internally). Phase 2: try securityLevel: 'sandbox' for a cleaner alternative.
vscode.dev / github.dev compat: build produces the web bundle, but end-to-end verification via @vscode/test-web is Phase 2 work.
Private GitHub user-attachment images (the TM-bridge workaround in the web app) are not yet bridged through vscode.authentication. Phase 3.