fix(theme): keep flat font properties in a mixed configureFonts config

src/theme/__tests__/fonts.test.js

@@ -340,6 +340,50 @@ describe('configureFonts', () => {
     });
   });
 
+  it('applies flat properties to every variant when the config also has per-variant entries', () => {
+    mockPlatform('ios');
+    const { configureFonts, typescale } = loadFonts();
+
+    const fonts = configureFonts({
+      config: {
+        fontFamily: 'NotoSans',
+        bodyLarge: {
+          fontSize: 18,
+        },
+      },
+    });
+
+    expect(fonts).toEqual({
+      ...Object.fromEntries(
+        Object.entries(typescale).map(([variantName, variantProperties]) => [
+          variantName,
+          { ...variantProperties, fontFamily: 'NotoSans' },
+        ])
+      ),
+      bodyLarge: {
+        ...typescale.bodyLarge,
+        fontFamily: 'NotoSans',
+        fontSize: 18,
+      },
+    });
+  });
+
+  it('does not add flat properties of a mixed config as typescale variants', () => {
+    mockPlatform('ios');
+    const { configureFonts } = loadFonts();
+
+    const fonts = configureFonts({
+      config: {
+        fontFamily: 'NotoSans',
+        bodyLarge: {
+          fontSize: 18,
+        },
+      },
+    });
+
+    expect(fonts.fontFamily).toBeUndefined();
+  });
+
   it('should be deterministic', () => {
     mockPlatform('ios');
     const { configureFonts } = loadFonts();

src/theme/fonts.tsx

@@ -13,34 +13,41 @@ function configureFontsConfig(
     return typescale;
   }
 
-  const isFlatConfig = Object.values(config).every(
-    (value) => typeof value !== 'object'
-  );
-
-  if (isFlatConfig) {
-    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-    return Object.fromEntries(
-      Object.entries(typescale).map(([variantName, variantProperties]) => [
-        variantName,
-        { ...variantProperties, ...config },
-      ])
-    ) as Typescale;
+  // A config entry is either a whole variant (an object, e.g. `bodyLarge: { fontSize: 18 }`)
+  // or a single font property shared by every variant (e.g. `fontFamily: 'NotoSans'`).
+  // Both may appear in the same config, so they are collected separately instead of
+  // classifying the config as a whole.
+  const sharedProperties: Record<string, unknown> = {};
+  const variantOverrides: Record<string, object> = {};
+
+  for (const [key, value] of Object.entries(config)) {
+    if (typeof value === 'object' && value !== null) {
+      variantOverrides[key] = value;
+    } else {
+      sharedProperties[key] = value;
+    }
   }
 
   const typescaleByVariant: Partial<
     Record<string, Typescale[keyof Typescale]>
   > = typescale;
 
-  return Object.assign(
-    {},
-    typescale,
-    ...Object.entries(config).map(([variantName, variantProperties]) => ({
-      [variantName]: {
+  const variantNames = new Set([
+    ...Object.keys(typescale),
+    ...Object.keys(variantOverrides),
+  ]);
+
+  // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+  return Object.fromEntries(
+    Array.from(variantNames, (variantName) => [
+      variantName,
+      {
         ...typescaleByVariant[variantName],
-        ...variantProperties,
+        ...sharedProperties,
+        ...variantOverrides[variantName],
       },
-    }))
-  );
+    ])
+  ) as Typescale;
 }
 
 export default function configureFonts(params?: {
@@ -51,6 +58,11 @@ export default function configureFonts(params?: {
   config?: Partial<Record<TypescaleKey, Partial<TypescaleStyle>>>;
 }): Typescale;
 // eslint-disable-next-line no-redeclare
+export default function configureFonts(params: {
+  config: Partial<TypescaleStyle> &
+    Partial<Record<TypescaleKey, Partial<TypescaleStyle>>>;
+}): Typescale;
+// eslint-disable-next-line no-redeclare
 export default function configureFonts(params: {
   config: Record<string, TypescaleStyle>;
 }): Typescale & { [key: string]: TypescaleStyle };