Harper is an offline, open-source grammar checker built in Rust + WebAssembly by Automattic (30,000+ users across VS Code, Chrome, Firefox, Neovim, and Obsidian). Lints documents in under 10ms using less than 1/50th of LanguageTool's memory with zero AI/ML. Active contributor fixing suggestion replacement and cursor preservation bugs across Draft.js, Lexical, Slate, ProseMirror, textarea, and generic contenteditable editors with full Playwright E2E test coverage. See merged PRs below for details.