docs: add EditorConfig integration research and update TODO

- Created todo/research/editorconfig-integration.md with detailed plan
- Added EditorConfig feature to Medium Priority in TODO.md
- Marked DirectWrite Font Variant Bug as fixed

todo/TODO.md

@@ -7,15 +7,19 @@
 
 ## Medium Priority
 
-- [ ] **DirectWrite Font Variant Bug** - Cascadia Mono Light breaks rendering
-  - Known Scintilla limitation with WSS font family model
-  - Not fixed in 5.5.8 - may need custom patch
-  - See: https://github.com/notepad-plus-plus/notepad-plus-plus/issues/12602
+- [x] **DirectWrite Font Variant Bug** - ✅ Fixed with ResolveFontFace() patch
+  - Uses IDWriteGdiInterop to resolve font face → family + weight/style/stretch
+  - See [scintilla/np3_patches/001_directwrite_font_resolution.md](../scintilla/np3_patches/001_directwrite_font_resolution.md)
 - [ ] **SVG Toolbar Icons** - Resolution-independent toolbar with dark/light mode
   - Auto-adapts to any DPI (100%-300%)
   - Dark/light mode color switching
   - Keep classic bitmap as fallback (ToolBarTheme 0/1)
   - See [research/svg-toolbar.md](research/svg-toolbar.md)
+- [ ] **EditorConfig Integration** - Apply `.editorconfig` settings on file open
+  - Use [editorconfig-core-c](https://github.com/editorconfig/editorconfig-core-c) library
+  - Support: indent_style, indent_size, tab_width, end_of_line, charset
+  - Effort: ~2-4 weeks
+  - See [research/editorconfig-integration.md](research/editorconfig-integration.md)
 - [ ] Installer testing on various Windows versions
 - [ ] Language file updates
 

todo/research/editorconfig-integration.md

@@ -0,0 +1,89 @@
+# EditorConfig Integration Research
+
+## Overview
+
+**EditorConfig** maintains consistent coding styles across editors and IDEs by reading `.editorconfig` files from the project directory tree.
+
+- Website: https://editorconfig.org/
+- Spec: https://spec.editorconfig.org/
+- C Library: https://github.com/editorconfig/editorconfig-core-c
+
+## Supported Properties
+
+| Property | Values | NP3 Scintilla API |
+|----------|--------|-------------------|
+| `indent_style` | `tab`, `space` | `SCI_SETUSETABS` |
+| `indent_size` | integer | `SCI_SETINDENT` |
+| `tab_width` | integer | `SCI_SETTABWIDTH` |
+| `end_of_line` | `lf`, `cr`, `crlf` | `SCI_SETEOLMODE` |
+| `charset` | `utf-8`, `latin1`, etc. | Encoding system |
+| `trim_trailing_whitespace` | `true`, `false` | On-save hook |
+| `insert_final_newline` | `true`, `false` | On-save hook |
+
+## Implementation Approach: editorconfig-core-c
+
+### Dependencies
+
+```
+editorconfig-core-c (BSD-2-Clause)
+├── PCRE2 (for glob matching) - Already in NP3 via Oniguruma? Check compatibility
+└── CMake (build only)
+```
+
+### Integration Points
+
+1. **On File Open** (`Notepad3.c` → `FileLoad()`)
+   - Walk up directory tree to find `.editorconfig` files
+   - Call `editorconfig_parse()` with full file path
+   - Apply settings via `Style_*` and `SciCall_*` functions
+
+2. **Settings Application** (`Styles.c` or new `EditorConfig.c`)
+   ```c
+   void ApplyEditorConfig(LPCWSTR filePath) {
+       editorconfig_handle eh = editorconfig_handle_init();
+       int err = editorconfig_parse(filePath, eh);
+       if (!err) {
+           int count = editorconfig_handle_get_name_value_count(eh);
+           for (int i = 0; i < count; i++) {
+               const char *name, *value;
+               editorconfig_handle_get_name_value(eh, i, &name, &value);
+               // Map to NP3 settings...
+           }
+       }
+       editorconfig_handle_destroy(eh);
+   }
+   ```
+
+3. **User Toggle** (Settings2 INI option)
+   - `EditorConfigSupport = 1` (default enabled)
+   - Menu: View → Apply EditorConfig (toggle)
+
+### Build Integration
+
+- Add `editorconfig-core-c` as Git submodule or NuGet package
+- Modify `scintilla.vcxproj` or create `editorconfig.vcxproj`
+- Link statically to avoid DLL dependency
+
+## Effort Estimate
+
+| Phase | Days | Description |
+|-------|------|-------------|
+| Library integration | 3-5 | Build editorconfig-core-c with MSVC, link to NP3 |
+| Core parsing | 2-3 | Hook file-open, call parser, get properties |
+| Settings mapping | 2-3 | Map EC properties to Scintilla/NP3 APIs |
+| On-save hooks | 1-2 | trim_trailing_whitespace, insert_final_newline |
+| UI/Settings | 1 | Toggle option, status bar indicator |
+| Testing | 2-3 | Various project structures, edge cases |
+| **Total** | **11-17** | ~2-4 weeks |
+
+## Current State
+
+- ✅ `.editorconfig` files syntax highlighted (`styleLexPROPS.c`)
+- ❌ Settings not applied when opening files
+- 📋 GitHub feature request: Consider filing issue
+
+## References
+
+- Notepad++ EditorConfig plugin: https://github.com/editorconfig/editorconfig-notepad-plus-plus
+- VS Code native support (built-in)
+- Sublime Text: EditorConfig package