[system] Fix unnecessary styles created from sx (mui#33752)

packages/mui-styled-engine-sc/src/index.d.ts

@@ -21,6 +21,15 @@ export { default as StyledEngineProvider } from './StyledEngineProvider';
 export { default as GlobalStyles } from './GlobalStyles';
 export * from './GlobalStyles';
 
+/**
+ * For internal usage in `@mui/system` package
+ */
+// eslint-disable-next-line @typescript-eslint/naming-convention
+export function internal_processStyles(
+  tag: React.ElementType,
+  processor: (styles: any) => any,
+): void;
+
 // These are the same as the ones in @mui/styled-engine
 // CSS.PropertiesFallback are necessary so that we support spreading of the mixins. For example:
 // '@font-face'?: Fontface | Fontface[]

packages/mui-styled-engine-sc/src/index.js

@@ -36,6 +36,18 @@ export default function styled(tag, options) {
   return stylesFactory;
 }
 
+// eslint-disable-next-line @typescript-eslint/naming-convention
+export const internal_processStyles = (tag, processor) => {
+  // Styled-components attaches an instance to `componentStyle`.
+  // https://github.com/styled-components/styled-components/blob/da8151762dcf72735ffba358173d4c097f6d5888/packages/styled-components/src/models/StyledComponent.ts#L257
+  //
+  // The instance contains `rules` (the styles)
+  // https://github.com/styled-components/styled-components/blob/da8151762dcf72735ffba358173d4c097f6d5888/packages/styled-components/src/models/ComponentStyle.ts#L23
+  if (tag.componentStyle) {
+    tag.componentStyle.rules = processor(tag.componentStyle.rules);
+  }
+};
+
 export { ThemeContext, keyframes, css } from 'styled-components';
 export { default as StyledEngineProvider } from './StyledEngineProvider';
 export { default as GlobalStyles } from './GlobalStyles';

packages/mui-styled-engine/src/index.d.ts

@@ -11,6 +11,15 @@ export { default as StyledEngineProvider } from './StyledEngineProvider';
 export { default as GlobalStyles } from './GlobalStyles';
 export * from './GlobalStyles';
 
+/**
+ * For internal usage in `@mui/system` package
+ */
+// eslint-disable-next-line @typescript-eslint/naming-convention
+export function internal_processStyles(
+  tag: React.ElementType,
+  processor: (styles: any) => any,
+): void;
+
 export interface SerializedStyles {
   name: string;
   styles: string;

packages/mui-styled-engine/src/index.js

@@ -1,3 +1,4 @@
+/* eslint-disable no-underscore-dangle */
 import emStyled from '@emotion/styled';
 
 export default function styled(tag, options) {
@@ -25,6 +26,15 @@ export default function styled(tag, options) {
   return stylesFactory;
 }
 
+// eslint-disable-next-line @typescript-eslint/naming-convention
+export const internal_processStyles = (tag, processor) => {
+  // Emotion attaches all the styles as `__emotion_styles`.
+  // Ref: https://github.com/emotion-js/emotion/blob/16d971d0da229596d6bcc39d282ba9753c9ee7cf/packages/styled/src/base.js#L186
+  if (Array.isArray(tag.__emotion_styles)) {
+    tag.__emotion_styles = processor(tag.__emotion_styles);
+  }
+};
+
 export { ThemeContext, keyframes, css } from '@emotion/react';
 export { default as StyledEngineProvider } from './StyledEngineProvider';
 export { default as GlobalStyles } from './GlobalStyles';

packages/mui-system/src/createStyled.js

@@ -1,4 +1,5 @@
-import styledEngineStyled from '@mui/styled-engine';
+/* eslint-disable no-underscore-dangle */
+import styledEngineStyled, { internal_processStyles as processStyles } from '@mui/styled-engine';
 import { getDisplayName } from '@mui/utils';
 import createTheme from './createTheme';
 import propsToClassKey from './propsToClassKey';
@@ -82,7 +83,17 @@ export default function createStyled(input = {}) {
     slotShouldForwardProp = shouldForwardProp,
     styleFunctionSx = defaultStyleFunctionSx,
   } = input;
+
+  const systemSx = (props) => {
+    const theme = isEmpty(props.theme) ? defaultTheme : props.theme;
+    return styleFunctionSx({ ...props, theme });
+  };
+  systemSx.__mui_systemSx = true;
+
   return (tag, inputOptions = {}) => {
+    // Filter out the `sx` style function from the previous styled component to prevent unnecessary styles generated by the composite components.
+    processStyles(tag, (styles) => styles.filter((style) => !style?.__mui_systemSx));
+
     const {
       name: componentName,
       slot: componentSlot,
@@ -131,7 +142,6 @@ export default function createStyled(input = {}) {
             // On the server Emotion doesn't use React.forwardRef for creating components, so the created
             // component stays as a function. This condition makes sure that we do not interpolate functions
             // which are basically components used as a selectors.
-            // eslint-disable-next-line no-underscore-dangle
             return typeof stylesArg === 'function' && stylesArg.__emotion_real !== stylesArg
               ? ({ theme: themeInput, ...other }) => {
                   return stylesArg({
@@ -176,10 +186,7 @@ export default function createStyled(input = {}) {
       }
 
       if (!skipSx) {
-        expressionsWithDefaultTheme.push((props) => {
-          const theme = isEmpty(props.theme) ? defaultTheme : props.theme;
-          return styleFunctionSx({ ...props, theme });
-        });
+        expressionsWithDefaultTheme.push(systemSx);
       }
 
       const numOfCustomFnsApplied = expressionsWithDefaultTheme.length - expressions.length;
@@ -194,7 +201,6 @@ export default function createStyled(input = {}) {
         // On the server Emotion doesn't use React.forwardRef for creating components, so the created
         // component stays as a function. This condition makes sure that we do not interpolate functions
         // which are basically components used as a selectors.
-        // eslint-disable-next-line no-underscore-dangle
         styleArg.__emotion_real !== styleArg
       ) {
         // If the type is function, we need to define the default theme.

packages/mui-system/src/createStyled.test.js

@@ -1,5 +1,6 @@
 import * as React from 'react';
 import { expect } from 'chai';
+import { spy } from 'sinon';
 import { ThemeProvider, createTheme } from '@mui/material/styles';
 import { createRenderer } from 'test/utils';
 import createStyled from './createStyled';
@@ -51,6 +52,44 @@ describe('createStyled', () => {
     });
   });
 
+  describe('composition', () => {
+    it('should call styleFunctionSx once', () => {
+      const styled = createStyled();
+      const spySx = spy();
+      const Child = styled('div')({});
+
+      render(<Child sx={spySx} />);
+
+      expect(spySx.callCount).to.equal(2); // React 18 renders twice in strict mode.
+    });
+
+    it('should still call styleFunctionSx once', () => {
+      const styled = createStyled();
+      const spySx = spy();
+      const Child = styled('div')({});
+      const Parent = styled(Child)({});
+
+      render(<Parent sx={spySx} />);
+
+      expect(spySx.callCount).to.equal(2); // React 18 renders twice in strict mode.
+    });
+
+    it('both child and parent still accept `sx` prop', () => {
+      const styled = createStyled();
+      const Child = styled('div')({});
+      const Parent = styled(Child)({});
+
+      const { container } = render(
+        <React.Fragment>
+          <Parent sx={{ color: 'rgb(0, 0, 255)' }} />
+          <Child sx={{ color: 'rgb(255, 0, 0)' }} />
+        </React.Fragment>,
+      );
+      expect(container.firstChild).toHaveComputedStyle({ color: 'rgb(0, 0, 255)' });
+      expect(container.lastChild).toHaveComputedStyle({ color: 'rgb(255, 0, 0)' });
+    });
+  });
+
   describe('styles', () => {
     it('styles of pseudo classes of variants are merged', () => {
       const theme = createTheme({