ADR 0180: Custom global UI type scale

• Status: Accepted (amended)
• Date: 2026-09-08
• Baseline: 0.4.16
• Protocol: unchanged (renderer-owned AppSettings JSON field)
• Storage schema: unchanged (optional AppSettings.fontScale)
• Related: D343, ADR 0083, 04-ux/06-settings-ia.md, 04-ux/07-ui-design-system.md

Context

The --text-* ramp is a frozen set of product sizes (--text-base is 14px, with denser chrome and larger headings). The application menu exposes Zoom In / Zoom Out / Reset Zoom, but those scale the entire window, including layout and controls. Users who want denser or larger type have no remembered preference that keeps the relative steps between body, chrome, and headings.

An earlier draft of this decision stored a reading-only px value and remapped only the transcript and composer. That left sidebar and settings at the product size, required the user to pick a px number, and did not scale surfaces that already use different steps of the same ramp.

Decision

1. Settings → General → Appearance gains a Font size row under Font. Four Starbucks-style cup presets (Tall 85% / Grande 100% / Venti 115% / Trenta 125%; zh-CN: 中杯 / 大杯 / 超大杯 / 超超大杯) sit above a percentage slider. Valid values are 80%–150% in 2.5% steps. Absent means 100%. The UI never asks for a px value.

2. Persistence is optional AppSettings.fontScale (1 = product ramp). Invalid writes are rejected at the renderer edge; reads clamp through normalizeFontScale. An unreleased leftover fontSize px field, if present without fontScale, migrates as px / 14. No host protocol or storage schema version bump.

3. Application is global. The renderer sets --font-scale on document.documentElement. Every --text-* token and --leading-row is calc(<product px> * var(--font-scale)), so body, chrome, headings, code, sidebar, and settings stay in proportion without a reload. Shared Lucide wrappers size glyphs with calc(<px> * var(--font-scale)) so toolbar, sidebar, composer, and settings icons track the same multiplier.

4. Zoom stays independent. Menu and shortcut Zoom In / Zoom Out / Reset Zoom continue to scale the whole window. Type scale and zoom compose.

Consequences

• One control scales every --text-* consumer; places that are already smaller or larger than body stay relatively smaller or larger.
• Users do not pick a px number that only matches --text-base.
• Large scales also enlarge compact chrome, including Lucide glyphs. Rows whose height is a fixed px value (not --leading-row) may still feel tighter; the 150% cap bounds that.

Alternatives

• Reading-only remap of .thread-wrap / .composer-dock: keeps chrome compact, but users then have two type sizes in one window and still need a px number for the body step.
• Zoom In/Out only: already exists; it scales chrome together with text and is not a type preference.
• Change html { font-size } and switch the ramp to rem: larger refactor; the product ramp is px tokens and the token guard forbids raw rem in component CSS.