Code change:
<del> /** * Copyright (c) Facebook, Inc. and its affiliates. * * This source code is licensed under the MIT license found in the * LICENSE file in the root directory of this source tree. * * @emails react-core */ 'use strict'; describe('ReactDOMShorthandCSSPropertyCollision', () => { let ReactFeatureFlags; let React; let ReactDOM; beforeEach(() => { jest.resetModules(); ReactFeatureFlags = require('shared/ReactFeatureFlags'); ReactFeatureFlags.warnAboutShorthandPropertyCollision = true; React = require('react'); ReactDOM = require('react-dom'); }); it('should warn for conflicting CSS shorthand updates', () => { const container = document.createElement('div'); ReactDOM.render(<div style={{font: 'foo', fontStyle: 'bar'}} />, container); expect(() => ReactDOM.render(<div style={{font: 'foo'}} />, container), ).toErrorDev( 'Warning: Removing a style property during rerender (fontStyle) ' + 'when a conflicting property is set (font) can lead to styling ' + "bugs. To avoid this, don't mix shorthand and non-shorthand " + 'properties for the same value; instead, replace the shorthand ' + 'with separate values.' + 'n    in div (at **)', ); // These updates are OK and don't warn: ReactDOM.render(<div style={{font: 'qux', fontStyle: 'bar'}} />, container); ReactDOM.render(<div style={{font: 'foo', fontStyle: 'baz'}} />, container); expect(() => ReactDOM.render( <div style={{font: 'qux', fontStyle: 'baz'}} />, container, ), ).toErrorDev( 'Warning: Updating a style property during rerender (font) when ' + 'a conflicting property is set (fontStyle) can lead to styling ' + "bugs. To avoid this, don't mix shorthand and non-shorthand " + 'properties for the same value; instead, replace the shorthand ' + 'with separate values.' + 'n    in div (at **)', ); expect(() => ReactDOM.render(<div style={{fontStyle: 'baz'}} />, container), ).toErrorDev( 'Warning: Removing a style property during rerender (font) when ' + 'a conflicting property is set (fontStyle) can lead to styling ' + "bugs. To avoid this, don't mix shorthand and non-shorthand " + 'properties for the same value; instead, replace the shorthand ' + 'with separate values.' + 'n    in div (at **)', ); // A bit of a special case: backgroundPosition isn't technically longhand // (it expands to backgroundPosition{X,Y} but so does background) ReactDOM.render( <div style={{background: 'yellow', backgroundPosition: 'center'}} />, container, ); expect(() => ReactDOM.render(<div style={{background: 'yellow'}} />, container), ).toErrorDev( 'Warning: Removing a style property during rerender ' + '(backgroundPosition) when a conflicting property is set ' + "(background) can lead to styling bugs. To avoid this, don't mix " + 'shorthand and non-shorthand properties for the same value; ' + 'instead, replace the shorthand with separate values.' + 'n    in div (at **)', ); ReactDOM.render( <div style={{background: 'yellow', backgroundPosition: 'center'}} />, container, ); // But setting them  at the same time is OK: ReactDOM.render( <div style={{background: 'green', backgroundPosition: 'top'}} />, container, ); expect(() => ReactDOM.render(<div style={{backgroundPosition: 'top'}} />, container), ).toErrorDev( 'Warning: Removing a style property during rerender (background) ' + 'when a conflicting property is set (backgroundPosition) can lead ' + "to styling bugs. To avoid this, don't mix shorthand and " + 'non-shorthand properties for the same value; instead, replace the ' + 'shorthand with separate values.' + 'n    in div (at **)', ); // A bit of an even more special case: borderLeft and borderStyle overlap. ReactDOM.render( <div style={{borderStyle: 'dotted', borderLeft: '1px solid red'}} />, container, ); expect(() => ReactDOM.render(<div style={{borderLeft: '1px solid red'}} />, container), ).toErrorDev( 'Warning: Removing a style property during rerender (borderStyle) ' + 'when a conflicting property is set (borderLeft) can lead to ' + "styling bugs. To avoid this, don't mix shorthand and " + 'non-shorthand properties for the same value; instead, replace the ' + 'shorthand with separate values.' + 'n    in div (at **)', ); expect(() => ReactDOM.render( <div style={{borderStyle: 'dashed', borderLeft: '1px solid red'}} />, container, ), ).toErrorDev( 'Warning: Updating a style property during rerender (borderStyle) ' + 'when a conflicting property is set (borderLeft) can lead to ' + "styling bugs. To avoid this, don't mix shorthand and " + 'non-shorthand properties for the same value; instead, replace the ' + 'shorthand with separate values.' + 'n    in div (at **)', ); // But setting them  at the same time is OK: ReactDOM.render( <div style={{borderStyle: 'dotted', borderLeft: '2px solid red'}} />, container, ); expect(() => ReactDOM.render(<div style={{borderStyle: 'dotted'}} />, container), ).toErrorDev( 'Warning: Removing a style property during rerender (borderLeft) ' + 'when a conflicting property is set (borderStyle) can lead to ' + "styling bugs. To avoid this, don't mix shorthand and " + 'non-shorthand properties for the same value; instead, replace the ' + 'shorthand with separate values.' + 'n    in div (at **)', ); }); }); </del>

Related discussion:
text: I a "bug" today that made me spent 1h figuring out what was going on: I'm using an external component that accepts a prop in order to set the of the root element, but this same component also accepts a prop which I wasn't passing, and by default, it was set to . The result: React didn't throw an error nor a warning, however, the resulting element in the DOM didn't contain either or , and since the element had a default coming from a CSS class, it took me a while to figure out why the color that I was passing wasn't being applied, and instead it was using the one from the CSS class. See: What I reported above was the case. I other cases as a bonus, as when I was playing with this they also seemed weird to me. On I define the same properties, but because I change the order, it works. On the toggleable ones, initially I can see the background, but after changing it never appears anymore. I'm not sure if I created those extra "test cases" correctly. My main concern is really around static1 not outputting anything on the console as a warning. Related issues: I wonder if ( should have covered this?
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Closing this issue after a prolonged period of inactivity. If this issue is still present in the latest release, please create a new issue with up-to-date information. Thank you!
it seems that the bot automatically closed the issue, but this still needs action. I took a time to investigate it, and I realized that the work Sophie has done covers this case, but there's just one problem: it's disabled under a . I built it locally and flipped the flag, and it works: I can see the helpful warning. It was flagged in , and as per the PR, the original intention was to unflag it on the next minor. Since then we had at least 4 new minor versions, but it is still flagged. Is there a special reason for that or was it just forgotten? tagging you since you this flag, so I'm hoping you can give context here.
Can we unflag this?
This was released.