fix(react-utilities): ensure slot className is saved from defaultProps (microsoft#35555)

Author: dmytrokirpa

change/@fluentui-react-utilities-5dc9804c-af24-4bc7-bcea-db65f03e0970.json

@@ -0,0 +1,7 @@
+{
+  "type": "patch",
+  "comment": "fix: ensure slot className is saved from defaultProps",
+  "packageName": "@fluentui/react-utilities",
+  "email": "[email protected]",
+  "dependentChangeType": "patch"
+}

packages/react-components/react-utilities/src/compose/slot.test.tsx

@@ -98,6 +98,22 @@ describe('slot', () => {
     });
   });
 
+  it('saves the original className from defaultProps', () => {
+    const props: TestProps = { slotA: {} };
+    const resolvedProps = slot.optional(props.slotA, {
+      elementType: 'div',
+      defaultProps: { className: 'defaultClass' },
+    });
+    if (resolvedProps) {
+      resolvedProps.className = [resolvedProps.className, 'addedClass'].join(' ');
+    }
+    expect(resolvedProps).toEqual({
+      [SLOT_ELEMENT_TYPE_SYMBOL]: 'div',
+      [SLOT_CLASS_NAME_PROP_SYMBOL]: 'defaultClass',
+      className: 'defaultClass addedClass',
+    });
+  });
+
   describe('.resolveShorthand', () => {
     it('resolves a string', () => {
       expect(slot.resolveShorthand('hello')).toEqual({ children: 'hello' });

packages/react-components/react-utilities/src/compose/slot.ts

@@ -40,7 +40,7 @@ export function always<Props extends UnknownSlotProps>(
     ...defaultProps,
     ...props,
     [SLOT_ELEMENT_TYPE_SYMBOL]: elementType,
-    [SLOT_CLASS_NAME_PROP_SYMBOL]: props?.className,
+    [SLOT_CLASS_NAME_PROP_SYMBOL]: props?.className || defaultProps?.className,
   } as SlotComponentType<Props>;
 
   if (props && typeof props.children === 'function') {