iOS Fabric: fontFamily (PostScript name) + explicit fontWeight resolves to the heaviest face — elvis-operator typo in RCTFontUtils.mm

Description

On iOS/Fabric in React Native 0.86.2, any <Text> style that combines a PostScript-named fontFamily with an explicit non-default fontWeight renders the heaviest face in the family instead of the requested weight.

// Renders Poppins-Medium (correct)
<Text style={{ fontFamily: 'Poppins-Medium' }} />

// Renders Poppins-Bold (bug — expected Poppins-Medium)
<Text style={{ fontFamily: 'Poppins-Medium', fontWeight: '500' }} />

// Renders Poppins-Bold (bug — expected Poppins-SemiBold)
<Text style={{ fontFamily: 'Poppins-SemiBold', fontWeight: '600' }} />

// Bare family name + weight resolves correctly (different code path)
<Text style={{ fontFamily: 'Poppins', fontWeight: '500' }} />  // correct

Verified via the rendered NSAttributedString font runs (not UIFont lookups): the fonts are registered and loadable; the resolver selects the wrong face.

Root cause

ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTFontUtils.mm (~line 364):

fontWeight = (fontWeight != 0.0) ?: RCTGetFontWeight(font);

The GNU ?: (elvis) operator assigns the boolean result of the comparison — 1.0 — for any explicit nonzero weight, rather than preserving the original fontWeight value. UIFontWeight 1.0 is the heaviest weight, so the subsequent family search selects the boldest face.

Suggested fix

fontWeight = (fontWeight != 0.0) ? fontWeight : RCTGetFontWeight(font);

(Note: this still cannot distinguish an explicit fontWeight: '400' from "no weight," since UIFontWeightRegular == 0.0 — a separate, pre-existing limitation.)

Impact

Any app pairing custom-font PostScript names with explicit weights (a common pattern, and the style many older codebases carry from pre-Fabric versions where the named face won) renders bold text across the board after upgrading. We hit this migrating a production app from 0.81.5 to 0.86.2 — every fontFamily: 'Poppins-<Face>' + matching fontWeight pair (~450 style sites) collapsed to Poppins-Bold. We are carrying a one-line patch-package fix of the ternary, which restores correct resolution for all pairings.

Steps to reproduce

1. Bundle a multi-weight custom font family (e.g. Poppins Regular/Medium/SemiBold/Bold) via UIAppFonts.
2. Render <Text style={{ fontFamily: 'Poppins-Medium', fontWeight: '500' }}>test</Text> on 0.86.2 with Fabric.
3. Inspect the rendered attributed string: the resolved font is Poppins-Bold.

Environment

React Native 0.86.2 (Fabric / new architecture), iOS 26.5 simulator + device, Expo SDK 57 prebuild (bare workflow equivalent). Regression vs 0.81.5 behavior.

ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTFontUtils.mm の364行目付近から始め、明示的なフォントウェイトがどのように選択されるかを調べます。Poppins PostScript-name の例で問題を再現し、レンダリングされた NSAttributedString のフォントランを調べます。明示的に指定されたデフォルト以外のウェイトが、最も太いフェイスではなく要求されたフェイスに解決されれば完了です。