iOS: nested Text with mixed fonts + lineHeight paints glyphs shifted/clipped under tail truncation — while onTextLayout/onLayout report correct positions

Description

A one-line (Arabic, RTL) containing nested spans where one span uses a different font/size (a fallback-font run), with an explicit lineHeight, paints its glyphs vertically shifted and bottom-clipped when the line is tail-truncated on iOS. Untruncated rows with identical structure paint correctly. The shift worsens with scroll depth in a ScrollView.

Reproduces across fonts: dramatic with a mushaf-grade face (KFGQPC Hafs — glyphs sink and bottom-clip), subtle but present with Scheherazade New (descender marks such as hamza+kasra paint slightly low). Severity scales with how far the font's ink extents diverge from its nominal metrics.

Critically: the shift is invisible to the layout APIs. onLayout and onTextLayout report identical geometry for shifted and correct rows — the repro measures both and prints the delta, which reads 0.0px while the painted output visibly diverges (the label is kept as a negative control). The defect appears to sit between text layout and rendering.

Plain single-font rows, flat rows, narrow rows, and RTL-aligned rows all paint correctly (mutations M1–M3 in the repro, all Δ 0.0 and visually clean) — the mixed-font nested run (M4) is the trigger.

Possibly related: #19653 (baseline offset computed from largest font-height in the string), #53092 (nested Text misalignment with lineHeight under the New Architecture).

Screenshots (also in the repo README): truncated rows sinking/clipping under Hafs+M4 vs their fitting pair rows.

Steps to reproduce

Reproduces across fonts: dramatic with a mushaf-grade face (KFGQPC Hafs — glyphs sink and bottom-clip), subtle but present with Scheherazade New (descender marks such as hamza+kasra paint slightly low). Severity scales with how far the font's ink extents diverge from its nominal metrics.

1. Clone https://github.com/muhasabahhub/rn-arabic-seat, npm install, npx expo start, open in Expo Go on a physical iPhone.
2. Select: tail (stock) + Hafs (mushaf) + M4 mixed-run.
3. Scroll the list. Rows come in pairs sharing their opening words — first fits, second truncates.
4. Compare pairs against the red guide: truncated rows paint shifted/bottom-clipped, worsening with scroll depth.
5. Note the Δ label reads 0.0px — onLayout/onTextLayout report identical seating while the paint diverges (negative control).
6. Toggle M4 off (or fonts to Amiri): all rows paint correctly, Δ 0.0 and visually clean.

React Native Version

0.86.3

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info

Expo SDK 57 managed project — envinfo equivalent below; full dependency table in the repo README.

 System:
    OS: Windows 11 10.0.26200
    CPU: (22) x64 Intel(R) Core(TM) Ultra 7 155H
    Memory: 3.80 GB / 15.59 GB
  Binaries:
    Node: 24.13.1 - C:\Program Files\nodejs\node.EXE
    npm: 11.19.0 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    expo: ~57.0.18 => 57.0.19
    expo-font: ~57.0.2 => 57.0.3
    react: 19.2.3 => 19.2.3
    react-native: 0.86.3 => 0.86.3
    react-native-safe-area-context: ~5.7.0 => 5.7.0


Device: iPhone 15, iOS 23.3.1, physical device via Expo Go. New Architecture (Fabric), Hermes.

Stacktrace or Logs

No crash or error output — this is a silent rendering defect. Nothing is logged.

The relevant "log" is the measurement built into the repro itself: each row pair prints a Δ label computed from onLayout + onTextLayout. For visibly shifted rows, the APIs report Δ 0.0px — i.e. the layout system emits no signal that the painted output has diverged. Sample readout while the defect is visible on screen:

  pair 3 (Hafs, tail, M4): reported Δ 0.0px — painted output shifted and bottom-clipped (screenshot 1)
  pair 5 (Hafs, tail, M4): reported Δ 0.0px — painted output shifted, worsens with scroll depth (screenshot 2)

The absence of any discrepancy in the layout events is part of the report: the defect is undetectable programmatically from the JS layer.

MANDATORY Reproducer

https://github.com/muhasabahhub/rn-arabic-seat

Screenshots and Videos

从必需的 rn-arabic-seat reproducer 开始，在实体 iPhone 上运行它，并将使用 Hafs 或 Scheherazade New 的 tail-truncated M4 行与对应的控制行进行比较。未指定任何 React Native 源文件或测试；完成的标准是，混合字体的 RTL 行在 tail truncation 下不再出现绘制偏移或底部被裁剪，同时报告的布局几何信息仍然正确。