在这里添加我的见解，因为这个帖子是关于这个主题的“go to”stackoverflow问题。

我的问题是在一个材质ui应用程序(早期阶段)

放置自定义主题提供程序是原因

当我做了一些计算强制渲染页面时 (其中一个组件“显示结果”取决于其他组件“输入部分”中设置的内容)。

一切都很好，直到我更新了强制“结果组件”重新呈现的“状态”。这里的主要问题是，我有一个材质ui主题(https://material-ui.com/customization/theming/#a-note-on-performance)在同一个渲染器(App.js / return..)作为“结果组件”，SummaryAppBarPure

解决方案是提升ThemeProvider一个级别(Index.js)，并在这里包装App组件，从而不强迫ThemeProvider重新计算和绘制/布局/回流。

之前

在App.js:

  return (
    <>
      <MyThemeProvider>
      <Container className={classes.appMaxWidth}>

        <SummaryAppBarPure
//...

在index.js

ReactDOM.render(
  <React.StrictMode>
      <App />
//...

后

在App.js:

return (
    <>
      {/* move theme to index. made reflow problem go away */}
      {/* <MyThemeProvider> */}
      <Container className={classes.appMaxWidth}>

        <SummaryAppBarPure
//...

在index.js

ReactDOM.render(
  <React.StrictMode>
    <MyThemeProvider>
      <App />
//...

答案是，这是新Chrome浏览器的一个功能，如果网页在执行JS时导致过多的浏览器回流，它会提醒你。详情请参阅

在Chrome控制台中Network选项卡下查找加载时间最长的脚本。

在我的例子中，有一组Angular add on脚本，我已经包含了，但还没有在应用中使用:

<script src="//cdnjs.cloudflare.com/ajax/libs/angular-ui-router/0.2.8/angular-ui-router.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/angular-ui-utils/0.1.1/angular-ui-utils.min.js"></script>
<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.9/angular-animate.min.js"></script>
<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.3.9/angular-aria.min.js"></script>

只有这些JavaScript文件的加载时间比“长时间运行任务”错误指定的时间长。

所有这些文件都在我的其他网站上运行，没有产生错误，但我在一个几乎没有任何功能的新web应用程序上得到了这个“长时间运行任务”错误。删除后错误立即停止。

我最好的猜测是，这些Angular add on正在递归地寻找DOM中越来越深的部分来寻找它们的开始标签——没有找到，它们必须在退出之前遍历整个DOM，这比Chrome预期的时间要长——因此出现了警告。

这不是一个错误，只是一个简单的消息。执行此消息更改 <！DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">(示例) 来 <！DOCTYPE html>(Firefox源代码期望此) 该消息显示在谷歌Chrome 74和Opera 60。改变后它是清晰的，0 verbose。 解决方法

不管怎样，这是我在遇到

[Violation] Forced reflow while executing JavaScript took <N>ms

警告。所讨论的页面是由用户内容生成的，因此我对DOM的大小实际上没有多大影响。在我的例子中，问题是一个有两列的表，可能有数百甚至数千行。(没有按需行加载实现，对不起!)

使用jQuery，在按下键时，页面选择一组行并切换它们的可见性。我注意到，在该集合上使用toggle()比显式使用hide() & show()更容易触发警告。

有关此特定性能场景的更多详细信息，请参见本文。

正如大家提到的，这些只是警告。然而，如果您热衷于解决这些问题(这是应该的)，那么您需要首先确定导致警告的原因。没有一个原因可以导致强制回流警告。 有人列出了一些可能的选择。您可以跟随讨论了解更多信息。 以下是可能原因的要点:

What forces layout / reflow All of the below properties or methods, when requested/called in JavaScript, will trigger the browser to synchronously calculate the style and layout*. This is also called reflow or layout thrashing, and is common performance bottleneck. Element Box metrics elem.offsetLeft, elem.offsetTop, elem.offsetWidth, elem.offsetHeight, elem.offsetParent elem.clientLeft, elem.clientTop, elem.clientWidth, elem.clientHeight elem.getClientRects(), elem.getBoundingClientRect() Scroll stuff elem.scrollBy(), elem.scrollTo() elem.scrollIntoView(), elem.scrollIntoViewIfNeeded() elem.scrollWidth, elem.scrollHeight elem.scrollLeft, elem.scrollTop also, setting them Focus elem.focus() can trigger a double forced layout (source) Also… elem.computedRole, elem.computedName elem.innerText (source) getComputedStyle window.getComputedStyle() will typically force style recalc (source) window.getComputedStyle() will force layout, as well, if any of the following is true: The element is in a shadow tree There are media queries (viewport-related ones). Specifically, one of the following: (source) * min-width, min-height, max-width, max-height, width, height * aspect-ratio, min-aspect-ratio, max-aspect-ratio device-pixel-ratio, resolution, orientation The property requested is one of the following: (source) height, width * top, right, bottom, left * margin [-top, -right, -bottom, -left, or shorthand] only if the margin is fixed. * padding [-top, -right, -bottom, -left, or shorthand] only if the padding is fixed. * transform, transform-origin, perspective-origin * translate, rotate, scale * webkit-filter, backdrop-filter * motion-path, motion-offset, motion-rotation * x, y, rx, ry window window.scrollX, window.scrollY window.innerHeight, window.innerWidth window.getMatchedCSSRules() only forces style Forms inputElem.focus() inputElem.select(), textareaElem.select() (source) Mouse events mouseEvt.layerX, mouseEvt.layerY, mouseEvt.offsetX, mouseEvt.offsetY (source) document doc.scrollingElement only forces style Range range.getClientRects(), range.getBoundingClientRect() SVG Quite a lot; haven't made an exhaustive list , but Tony Gentilcore's 2011 Layout Triggering List pointed to a few. contenteditable Lots & lots of stuff, …including copying an image to clipboard (source)

点击这里查看更多信息。

另外，这里是原始问题的Chromium源代码，以及关于警告的性能API的讨论。

编辑:谷歌还有一篇关于如何在PageSpeed Insight上最小化布局回流的文章。它解释了什么是浏览器回流:

Reflow is the name of the web browser process for re-calculating the positions and geometries of elements in the document, for the purpose of re-rendering part or all of the document. Because reflow is a user-blocking operation in the browser, it is useful for developers to understand how to improve reflow time and also to understand the effects of various document properties (DOM depth, CSS rule efficiency, different types of style changes) on reflow time. Sometimes reflowing a single element in the document may require reflowing its parent elements and also any elements which follow it.

此外，它还解释了如何将其最小化:

Reduce unnecessary DOM depth. Changes at one level in the DOM tree can cause changes at every level of the tree - all the way up to the root, and all the way down into the children of the modified node. This leads to more time being spent performing reflow. Minimize CSS rules, and remove unused CSS rules. If you make complex rendering changes such as animations, do so out of the flow. Use position-absolute or position-fixed to accomplish this. Avoid unnecessary complex CSS selectors - descendant selectors in particular - which require more CPU power to do selector matching.