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Android Fabric text measurement uses Typeface.DEFAULT instead of the system default used for rendering, clipping last glyphs with bold system fonts

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Bewertung

Schwierigkeit
4/5
Geschätzter Aufwand
3-5 Tage
Anfängerfreundlichkeit
55/100
Issue-Typ
Bug
Klarheit
Größtenteils klar
Aktivitätsstatus
Ruhig
Tech-Stack
android, react-native
Bereich
mobile-dev

Rechercherichtung

Start with TextLayoutManager.updateTextPaint() and compare its typeface handling with the ReactTextView rendering path. Reproduce the issue using an Android device with a bold or replacement system font and an auto-width Text component. Done means measurement uses the same default typeface as rendering and the final glyph is no longer clipped.

Vom Indexierungsmodell aus dem Issue-Text verfasst.

Beschreibung

Needs: Attention Needs: Repro

Summary

When Android system default font is bolded/replaced (e.g. Xiaomi MiSans Pro Bold / 'full bold' setting), auto-width components are measured with Typeface.DEFAULT while rendering uses the TextView default typeface which inherits the system font configuration. The measured width is therefore too small, and the last glyph can be clipped or missing.

Example: English renders as Englis on Xiaomi with the system-wide bold font enabled.

Environment

  • React Native 0.78.3 (Fabric / Bridgeless)
  • Android 16 (API 36), Xiaomi HyperOS with system font set to MiSans Pro Bold
  • Reproduced on multiple Xiaomi devices

Root cause

TextLayoutManager.updateTextPaint() resets the paint typeface to null (Typeface.DEFAULT) when no explicit font family/weight/style is set, while ReactTextView renders with the TextView default typeface which carries the system bold/replacement config. Different metrics and advances mean measurement is narrower than what is actually drawn.

On device, same 48px text:

ascent descent line height
measurement paint (Typeface.DEFAULT) -45 3 1.0em
rendering TextView (system bold) -50 14 1.33em

English (7 glyphs): measured width 53.33dp (only 6 glyphs laid out), drawn width 56.67dp (7 glyphs). The final h is dropped.

Suggested fix

Make the measurement typeface the same source as rendering when no explicit font family/weight/style is set. A local patch that uses the system/default TextView typeface for measurement fixes the issue and was verified on 5 Android devices.

Additional context

This analysis and the local fix were produced with the assistance of an AI coding agent (Codex). Please review independently.

Related

  • #49886 (text metrics mismatch / descender clipping)
Vorherrschende Sprache
C++
Sterne
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