[Webkit-unassigned] [Bug 235322] New: Adding iframe flushes microtasks synchronously with dirty stack

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=235322

            Bug ID: 235322
           Summary: Adding iframe flushes microtasks synchronously with
                    dirty stack
           Product: WebKit
           Version: Safari 15
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: dan.abramov at gmail.com

Created attachment 449401

  --> https://bugs.webkit.org/attachment.cgi?id=449401&action=review

repro

Microtasks are supposed to run on a clean stack. However, Safari flushes microtasks synchronously if you append an iframe to the page. This breaks expectations for libraries like React/Preact, and possibly others:

- https://github.com/facebook/react/issues/22459
- https://github.com/preactjs/preact/issues/2814


Here is a simple repro:

```
queueMicrotask(() => {
  console.log("--- in microtask ---");
});
console.log("will add iframe");
const iframe = document.createElement("iframe");
iframe.src = "localhost";
document.body.appendChild(iframe);
console.log("did add iframe");
```

You need to run it with http server to repro. It doesn't repro from file://.

Expected output:

```
will add iframe
did add iframe
--- in microtask ---
```

Safari output:

```
will add iframe
--- in microtask ---
did add iframe
```

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