Add multi-window editor support

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Added
+
+- Multi-window editing with one independently autosaved presentation per editor window, per-editor presentation/debug windows, duplicate-file focusing, and cross-platform file-open routing
+
 ## [1.2.0] - 2026-08-24
 
 ### Added

CLAUDE.md

@@ -64,6 +64,13 @@ twig follows Electron's standard architecture:
 2. **Preload Script** (`src/preload/index.ts`): Bridge layer that safely exposes IPC handlers to the renderer via `contextBridge`
 3. **Renderer Process** (`src/renderer/`): Svelte application running in the browser context
 
+### Multi-Window Ownership (`src/main/editorWindowManager.ts`)
+
+- Every editor renderer owns exactly one committed presentation and may temporarily reserve one replacement path while opening or saving.
+- Canonical path identities prevent the same `.tb` file from being owned by multiple editor windows.
+- Presentation and debug windows are scoped to an owning editor; their IPC must always route through that owner.
+- Each editor runs in a separate renderer process, so the module-level Svelte state remains isolated per window.
+
 ### Database Layer (`src/main/db.ts`)
 
 - Uses better-sqlite3 for synchronous SQLite operations

README.md

@@ -35,7 +35,7 @@ Presentations are stored as `.tb` files — plain SQLite databases. No cloud req
 
 ## Status
 
-twig is in active development. It handles the essentials — editing, alignment guides, transitions, animations, custom fonts, backgrounds — and works well for day-to-day use.
+twig is in active development. It handles the essentials — editing, multi-window workflows, alignment guides, transitions, animations, custom fonts, backgrounds — and works well for day-to-day use.
 
 On the roadmap: templates, element grouping, and more transitions and animation types.
 

src/main/db/connection.ts

@@ -45,7 +45,7 @@ export function validateFilePath(filePath: unknown): asserts filePath is string
   if (!isAbsolute(filePath)) {
     throw new Error('File path must be absolute')
   }
-  if (!filePath.endsWith('.tb')) {
+  if (!filePath.toLowerCase().endsWith('.tb')) {
     throw new Error('Invalid file extension. Expected .tb file')
   }
   const normalized = normalize(filePath)