Custom Font Selection

This document describes the custom font selection feature in Ferrite, which allows users to choose their preferred fonts for the editor and configure CJK regional glyph preferences.

Overview

Ferrite supports three types of font configurations:

1. Built-in Fonts: Inter (proportional) and JetBrains Mono (monospace) are bundled with the application
2. Custom System Fonts: Users can select any font installed on their system
3. CJK Regional Preferences: Users can prioritize specific regional variants for CJK characters

Feature Details

Built-in Fonts

• Inter: A modern, clean proportional font used for general text
• JetBrains Mono: A monospace font optimized for code and technical documents

Both fonts include bold and italic variants for proper styling.

Custom System Font Selection

Users can select any system font from Settings → Appearance → Font.

How it works:

1. The system font list is enumerated using font-kit crate on application startup
2. Fonts are cached to avoid re-enumeration on each settings panel open
3. Choosing Custom sets EditorFont::Custom with an empty name until the user picks a row in the combo; font reload runs without a custom primary face until then (Inter stays active). This avoids immediately loading the first sorted family name, which can fail on some macOS setups where enumeration names do not resolve the same way as select_best_match / path reads (GitHub #133).
4. When a custom font is selected, it's loaded dynamically and added to the font fallback chain
5. The custom font name is stored in config.json

Empty custom names are normalized away when loading settings (Settings::sanitize).

Implementation detail: Custom font picker deferred load.

Limitations:

• Custom fonts don't have separate bold/italic variants loaded
• The OS may synthesize bold/italic styles depending on the font and platform
• Not all system fonts may be suitable for text editing

CJK Regional Glyph Preferences

CJK (Chinese, Japanese, Korean) fonts render the same Unicode code points differently based on regional conventions. This setting controls which regional font takes priority.

Available options:

Option	Priority Order	Description
Auto	KR → SC → TC → JP	Use system locale to determine
Korean	KR → SC → TC → JP	Prioritize Korean glyph variants
Simplified Chinese	SC → TC → KR → JP	Prioritize Simplified Chinese
Traditional Chinese	TC → SC → KR → JP	Prioritize Traditional Chinese
Japanese	JP → KR → SC → TC	Prioritize Japanese glyph variants

System fonts used:

Region	macOS	Windows	Linux
Korean	Apple SD Gothic Neo	Malgun Gothic	Noto Sans CJK KR, NanumGothic
Simplified Chinese	PingFang SC	Microsoft YaHei	Noto Sans CJK SC
Traditional Chinese	PingFang TC	Microsoft JhengHei	Noto Sans CJK TC
Japanese	Hiragino Sans	Yu Gothic, Meiryo	Noto Sans CJK JP

Implementation

Key Files

File	Purpose
src/fonts.rs	Font loading, system font enumeration, and runtime font reload
src/config/settings.rs	EditorFont enum with Custom variant, CjkFontPreference enum
src/ui/settings.rs	Settings UI for font picker and CJK preference dropdown
src/app/ (central_panel.rs, mod.rs)	Font reload when settings change; startup custom load

EditorFont Enum

pub enum EditorFont {
    Inter,           // Built-in proportional font
    JetBrainsMono,   // Built-in monospace font
    Custom(String),  // Custom system font name
}

CjkFontPreference Enum

pub enum CjkFontPreference {
    Auto,              // Use system locale
    Korean,            // Prioritize Korean glyphs
    SimplifiedChinese, // Prioritize SC glyphs
    TraditionalChinese,// Prioritize TC glyphs
    Japanese,          // Prioritize Japanese glyphs
}

Font Reload

Fonts are reloaded at runtime when settings change:

// In app.rs, when settings change:
if font_changed {
    fonts::reload_fonts(
        ctx,
        custom_font.as_deref(),
        settings.cjk_font_preference,
    );
}

Configuration

Font settings are stored in config.json:

{
  "font_family": "custom",           // or "inter" or "jetbrainsmono"
  "font_family": { "custom": "Arial" }, // when using custom font
  "cjk_font_preference": "auto"      // or "korean", "simplifiedchinese", etc.
}

Testing

• Test font picker shows system fonts
• Test selecting a custom font applies to editor
• Test CJK fonts render correct regional glyphs
• Test font settings persist across restart
• Test invalid custom font falls back gracefully

Related

• GitHub Issue: #15 - CJK regional glyph preferences
• Dependencies: font-kit 0.14.3 for system font enumeration