[whatwg/fullscreen] Upstream Document PiP changes to "fullscreen is supported" (PR #259)

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I think it makes sense to merge this after https://github.com/whatwg/html/pull/12397 because `Is Document Picture-in-Picture` is currently not exported.

- [ ] At least two implementers are interested (and none opposed):
   * Gecko
   * ...
- [x] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * https://wpt.fyi/results/document-picture-in-picture/pip-fullscreen.https.html
- [x] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chromium: already shipped
   * Gecko: already shipped
   * WebKit: open standard position https://github.com/WebKit/standards-positions/issues/41
- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: ...
- [x] The top of this comment includes a [clear commit message](https://github.com/whatwg/meta/blob/main/COMMITTING.md) to use.

Fixes https://github.com/whatwg/fullscreen/issues/258


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### :boom: Error: 422 Unprocessable Entity :boom: ###

[PR Preview](https://github.com/tobie/pr-preview#pr-preview) failed to build. _(Last tried on Apr 22, 2026, 12:05 PM UTC)_.

<details>
<summary>More</summary>


PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

:rotating_light: [Spec Generator](https://www.w3.org/publications/spec-generator/) - Spec Generator is the web service used to build bikeshed/ReSpec specs

:link: [Related URL](https://www.w3.org/publications/spec-generator/?type=bikeshed-spec&output=html&url=https%3A%2F%2Fraw.githubusercontent.com%2Fvinhill%2Ffullscreen%2Fe18ca6d324d3e6ba6d3d0365313e99efc8a3b66c%2Ffullscreen.bs&force=1&md-status=LS-PR&md-Text-Macro=PR-NUMBER%20259)

**Error output:**

```json
[
    {
        "lineNum": "114:35",
        "messageType": "link",
        "text": "Multiple possible 'removing steps' dfn refs.\nArbitrarily chose https://dom.spec.whatwg.org/#concept-node-remove-ext\nTo auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:\nspec:dom; type:dfn; text:removing steps\nspec:geolocation-element; type:dfn; for:ActivationBlockersMixin; text:removing steps\nspec:geolocation-element; type:dfn; for:HTMLGeolocationElement; text:removing steps\nspec:geolocation-element; type:dfn; for:PowerfulFeatureObserver; text:removing steps\n<a bs-line-number=\"114:35\" data-link-type=\"dfn\" data-lt=\"removing steps\">removing steps</a>"
    },
    {
        "lineNum": "157:31",
        "messageType": "link",
        "text": "No 'dfn' refs found for 'is document picture-in-picture' that are marked for export.\n  (Possible specs this could be from: document-picture-in-picture)\n<a bs-line-number=\"157:31\" data-link-type=\"dfn\" data-lt=\"Is Document Picture-in-Picture\">Is Document Picture-in-Picture</a>"
    },
    {
        "lineNum": null,
        "messageType": "failure",
        "text": "Did not generate, due to errors exceeding the allowed error level."
    }
]
```

_This seems to be an issue with the [Spec Generator](https://www.w3.org/publications/spec-generator/) service. PR Preview doesn't manage this service and so has no control over it. If you've identified an issue with it, you can [report the issue to the maintainers of Spec Generator](https://github.com/w3c/spec-generator/issues/new) directly. Please be courteous. Thank you!_

_If you don't have enough information above to solve the error by yourself or if the issue doesn't seem related to Spec Generator, you can [file an issue with PR Preview](https://github.com/tobie/pr-preview/issues/new?title=Unidentified%20Error&body=See%20whatwg/fullscreen%23259.)._

</details>

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fullscreen/pull/259?email_source=notifications&email_token=ACLYVMJIHVLZVQVT4SIEY5T4XCYTBA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTKNRXGA3TGMJQGCTHEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVVYHEX3POBSW4X3DNRUWG2Y

-- Commit Summary --

  * Upstream Document Picture-in-Picture changes to "fullscreen is supported"

-- File Changes --

    M fullscreen.bs (21)

-- Patch Links --

https://github.com/whatwg/fullscreen/pull/259.patch?email_source=notifications&email_token=ACLYVMNA7AKIBSRUXBEZD7T4XCYTBA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTKNRXGA3TGMJQGCTHEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVZYHEX3QMF2GG2C7MNWGSY3Lhttps://github.com/whatwg/fullscreen/pull/259.diff?email_source=notifications&email_token=ACLYVMN346DUTWHTFW4XWVL4XCYTBA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTKNRXGA3TGMJQGCTHEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVVYHEX3ENFTGMX3DNRUWG2Y
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