[base-ui] Fix mergeSlotProps className join order

packages/mui-base/src/utils/mergeSlotProps.test.ts

@@ -40,36 +40,65 @@ describe('mergeSlotProps', () => {
     expect(merged.props.prop4).to.equal('internal');
   });
 
-  it('joins all the class names in order from internal to external', () => {
-    const getSlotProps = () => ({
-      className: 'internal',
+  describe('it joins all class names in order from least to most important', () => {
+    it('when internal classNames from getSlotProps are included', () => {
+      const getSlotProps = () => ({
+        className: 'internal',
+      });
+
+      const additionalProps = {
+        className: 'additional',
+      };
+
+      const externalForwardedProps = {
+        className: 'externalForwarded',
+      };
+
+      const externalSlotProps = {
+        className: 'externalSlot',
+      };
+
+      const className = ['class1', 'class2'];
+
+      const merged = mergeSlotProps({
+        getSlotProps,
+        additionalProps,
+        externalForwardedProps,
+        externalSlotProps,
+        className,
+      });
+
+      expect(merged.props.className).to.equal(
+        'internal additional class1 class2 externalForwarded externalSlot',
+      );
     });
 
-    const additionalProps = {
-      className: 'additional',
-    };
+    it('when getSlotProps is not present', () => {
+      const additionalProps = {
+        className: 'additional',
+      };
 
-    const externalForwardedProps = {
-      className: 'externalForwarded',
-    };
+      const externalForwardedProps = {
+        className: 'externalForwarded',
+      };
 
-    const externalSlotProps = {
-      className: 'externalSlot',
-    };
+      const externalSlotProps = {
+        className: 'externalSlot',
+      };
 
-    const className = ['class1', 'class2'];
+      const className = ['class1', 'class2'];
 
-    const merged = mergeSlotProps({
-      getSlotProps,
-      additionalProps,
-      externalForwardedProps,
-      externalSlotProps,
-      className,
-    });
+      const merged = mergeSlotProps({
+        additionalProps,
+        externalForwardedProps,
+        externalSlotProps,
+        className,
+      });
 
-    expect(merged.props.className).to.equal(
-      'internal additional class1 class2 externalForwarded externalSlot',
-    );
+      expect(merged.props.className).to.equal(
+        'additional class1 class2 externalForwarded externalSlot',
+      );
+    });
   });
 
   it('merges the style props', () => {

packages/mui-base/src/utils/mergeSlotProps.ts

@@ -90,10 +90,10 @@ export function mergeSlotProps<
     // The simpler case - getSlotProps is not defined, so no internal event handlers are defined,
     // so we can simply merge all the props without having to worry about extracting event handlers.
     const joinedClasses = clsx(
+      additionalProps?.className,
+      className,
       externalForwardedProps?.className,
       externalSlotProps?.className,
-      className,
-      additionalProps?.className,
     );
 
     const mergedStyle = {

packages/mui-material-next/src/FilledInput/FilledInput.test.tsx

@@ -1,5 +1,6 @@
 import * as React from 'react';
 import { expect } from 'chai';
+import { ClassNames } from '@emotion/react';
 import { createRenderer, describeConformance } from '@mui-internal/test-utils';
 import { CssVarsProvider, extendTheme } from '@mui/material-next/styles';
 import FilledInput, { filledInputClasses as classes } from '@mui/material-next/FilledInput';
@@ -65,4 +66,36 @@ describe('<FilledInput />', () => {
     const root = getByTestId('test-input');
     expect(root).toHaveComputedStyle({ marginTop: '10px' });
   });
+
+  describe('Emotion compatibility', () => {
+    it('classes.root should overwrite built-in styles.', () => {
+      const { getByTestId } = render(
+        <ClassNames>
+          {({ css }) => (
+            <FilledInput data-testid="root" classes={{ root: css({ position: 'static' }) }} />
+          )}
+        </ClassNames>,
+      );
+      const input = getByTestId('root');
+
+      expect(getComputedStyle(input).position).to.equal('static');
+    });
+
+    it('className should overwrite classes.root and built-in styles.', () => {
+      const { getByTestId } = render(
+        <ClassNames>
+          {({ css }) => (
+            <FilledInput
+              data-testid="root"
+              className={css({ position: 'sticky' })}
+              classes={{ root: css({ position: 'static' }) }}
+            />
+          )}
+        </ClassNames>,
+      );
+      const input = getByTestId('root');
+
+      expect(getComputedStyle(input).position).to.equal('sticky');
+    });
+  });
 });

packages/mui-material-next/src/FormControl/FormControl.test.tsx

@@ -1,6 +1,7 @@
 import * as React from 'react';
 import { expect } from 'chai';
 import { spy } from 'sinon';
+import { ClassNames } from '@emotion/react';
 import { describeConformance, act, createRenderer, fireEvent } from '@mui-internal/test-utils';
 import FormControl, { formControlClasses as classes } from '@mui/material-next/FormControl';
 import FilledInput from '@mui/material-next/FilledInput';
@@ -444,4 +445,36 @@ describe('<FormControl />', () => {
       });
     });
   });
+
+  describe('Emotion compatibility', () => {
+    it('classes.root should overwrite built-in styles.', () => {
+      const { getByTestId } = render(
+        <ClassNames>
+          {({ css }) => (
+            <FormControl data-testid="root" classes={{ root: css({ display: 'inline' }) }} />
+          )}
+        </ClassNames>,
+      );
+      const root = getByTestId('root');
+
+      expect(getComputedStyle(root).display).to.equal('inline');
+    });
+
+    it('className should overwrite classes.root and built-in styles.', () => {
+      const { getByTestId } = render(
+        <ClassNames>
+          {({ css }) => (
+            <FormControl
+              data-testid="root"
+              className={css({ display: 'inline-block' })}
+              classes={{ root: css({ display: 'inline' }) }}
+            />
+          )}
+        </ClassNames>,
+      );
+      const root = getByTestId('root');
+
+      expect(getComputedStyle(root).display).to.equal('inline-block');
+    });
+  });
 });