Bug: <iframe /> onLoad is not getting triggered after the reload triggered by moving the DOM Node

When an `<iframe />` DOM Node is moved to a different place in the tree, Chrome triggers a reload. That reload does not notify event handlers registered via `onLoad=`. In other words: * If there is an event handler registered via `onLoad={myCallback}`, it **will not** be fired. * If there is an event handler registered via `iframe.addEventListener("load", myCallback, false)`, it **will** be fired. React version: 16.13.1 ## Steps To Reproduce Open a code example below and move the iframe **down** using arrow button next to it. https://codesandbox.io/s/serverless-surf-k1ypg?file=/src/App.js ## The current behavior The iframe does not say "Initialized" anymore. ## The expected behavior The iframe should say "Initialized". ## Other notes If you move `setIframeContent(e.currentTarget);` to `nativeLoad` and repeat the steps above, it works as expected. The bug does not occur in Firefox.