Bug 302102 – [Site Isolation] Incorrect origin for about:blank iframes

• Component: WebKit Process Model
• Status: RESOLVED FIXED

[Anthony Tarbinian]

Summary:
With site isolation turned on, when a cross origin iframe navigates back to about:blank it keeps its cross origin status. 

Steps To Reproduce:
1. Open a page to `about:blank`
2. Open the WebInspector
3. Run the following JS commands in the console:
```
let i = document.createElement('iframe'); document.body.appendChild(i);
i.src="https://example.com"
i.src="about:blank"
i.contentWindow.document.body.appendChild(i.contentWindow.document.createTextNode('some text'))
```

Results:
Observe how attempting to modify the DOM of the about:blank is blocked with a SecurityError: “SecurityError: Blocked a frame with origin "null" from accessing a cross-origin frame. Protocols, domains, and ports must match..”

According to the HTML spec (https://html.spec.whatwg.org/multipage/browsing-the-web.html#beginning-navigation):
> If url matches about:blank or is about:srcdoc, then:
> 
> Set documentState's origin to initiatorOriginSnapshot.
> 
> Set documentState's about base URL to initiatorBaseURLSnapshot.


Regression:
Problem reproducibly occurs on WebKit with site isolation enabled. With site isolation off, the DOM of the about:blank iframe is able to be modified by the main frame.

[Anthony Tarbinian]

<rdar://problem/160706600>

[Anthony Tarbinian]

Pull request: https://github.com/WebKit/WebKit/pull/52845

[EWS]

Committed 303299@main (84738405475e): <https://commits.webkit.org/303299@main>

Reviewed commits have been landed. Closing PR #52845 and removing active labels.

[Anthony Tarbinian]

Broke scrolling with site isolation (see https://bugs.webkit.org/show_bug.cgi?id=303451). Original commit was reverted, so re-opening this issue.

[Anthony Tarbinian]

Pull request: https://github.com/WebKit/WebKit/pull/54861