Fix custom font weight rendering heaviest face on iOS Fabric

[jensdev]

Summary:

On the New Architecture (Fabric), a custom font referenced by its PostScript / full font name (e.g. fontFamily: "Foo-Medium") combined with an explicit non-regular fontWeight renders at the heaviest available face (bold/black) instead of the requested weight. This is an iOS-only regression introduced in 0.83; 0.82 was correct. Android is unaffected.

Root cause. In RCTFontUtils.mm, RCTFontWithFontProperties() handles the case where the given fontFamily is actually a font name rather than a family name (fontNames.count == 0) and resolves the effective weight with:

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

fontWeight is a UIFontWeight (a double: Regular = 0.0, Medium = 0.23, Bold = 0.4, Black = 0.62). The Objective-C "Elvis" operator A ?: B evaluates to A itself when A is truthy — and here A is the comparison (fontWeight != 0.0), a BOOL. So whenever a weight was set (e.g. 0.23), fontWeight was reassigned to 1.0 (heavier than Black), and the subsequent "closest weight in the family" search always picked the heaviest face.

This was introduced by the automated implicit-bool-conversion sweep in #53591 (D81571883), which rewrote the original correct line fontWeight ?: RCTGetFontWeight(font) into (fontWeight != 0.0) ?: …. Making the truthiness check explicit is fine as a condition, but with ?: the left-hand side is also the returned value, so the numeric weight got replaced by the boolean.

Fix.

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

For a double, (A != 0.0) ? A : B is exactly equivalent to the original A ?: B, so this restores the 0.81/0.82 behavior while keeping the explicit != 0.0 form used elsewhere in the file. The no-weight case is unchanged: RCTResolveFontProperties fills an unspecified weight with UIFontWeightRegular (0.0), so the weight is still inferred from the font name via RCTGetFontWeight(font) in that case. The legacy (Paper) path in React/Views/RCTFont.mm uses a different construct and is not affected — consistent with his only reproducing on the New Architecture.

Changelog:

[iOS] [Fixed] - Custom fonts with an explicit fontWeight no longer render at the heaviest weight on the New Architecture

Test Plan:

Fixes #54934.

New Architecture, iOS, with a custom font bundled and referenced by its PostScript name,
e.g. <Text style={{ fontFamily: 'Foo-Medium', fontWeight: '500' }}>:

Version	Result
0.82	renders at the requested medium weight ✅
0.83 / main (before fix)	renders bold/black ❌
removing fontWeight on 0.83	renders correctly ✅ (confirms the weight branch is the culprit)

After this change the text renders at the requested weight on both the old and new architecture.

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