For #script-closable, clarify when a browsing context is created by script

[vinhill]

Introduce an "is created by web content" flag on browsing contexts to determine script-closability. This flag is set when a new top-level traversable is created as a result of window.open(), link navigation, or form submission.

In reference to #1866.

• At least two implementers are interested (and none opposed):
  • Gecko
  • WebKit (@annevk)
• Tests are written and can be reviewed and commented upon at:
  • https://phabricator.services.mozilla.com/D245035
  • I think the session history check is not testable, I filed The session history check of whether a browsing context is script-closable is not testable web-platform-tests/wpt#51933
• Implementation bugs are filed:
  • This is already implemented in Blink, Gecko and WebKit
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed:
  • I'm working on this though https://bugzilla.mozilla.org/show_bug.cgi?id=1316857
• The top of this comment includes a clear commit message to use.

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

cc @zcorpan

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/document-sequences.html ( diff )
/nav-history-apis.html ( diff )