Specify browser UI-initiated navigations better

[domenic]

In particular, they have a null sourceDocument. We need to handle that in various places in the specification.

Closes #9133.

In addition to @domfarolino and @noamr, review from @ADKaster or other Ladybird folks who might have run into this would be much appreciated.

• At least two implementers are interested (and none opposed):
  • This matches all implementations, as far as I can tell.
• Tests are written and can be reviewed and commented upon at:
  • This isn't testable in WPT: Testing browser-initiated navigation. web-platform-tests/wpt#16019
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: N/A
  • WebKit: N/A
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

/browsing-the-web.html ( diff )
/infrastructure.html ( diff )

[ADKaster]

or other Ladybird folks who might have run into this would be much appreciated.

cc @trflynn89 @shannonbooth :)

[annevk]

Looks good. This puts some pressure on Fetch getting its null client act together.

[domenic]

I will hold off on merging this until we have agreement that whatwg/fetch#1820 is the right way to fix the request's window. (For example, we might instead prefer callers to set it.)

[ADKaster]

Implementation musings:

Implementing this in LadybirdBrowser/ladybird#4515 caused one of my tests for the state of window.navigation.canGoBack after a BrowserUI initiated navigation to fail -- the current entry index got reset to 0. I think that's... correct? Because our test runner does this:

load(about:blank)
load(test file url)
<wait for test to complete>
load(about:blank)
load(test file url)
<wait for test to complete>

Though navigating to about:blank causing all previous state to go kaput seems odd, it's not like anyone who isn't trying to stress test the browser would do that during normal operation, right? ...