Exposing HTMLSourceElement IDL attributes
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Implement support for Element.toggleAttribute
Copy implementation following from [Firefox implementation](https://bugzilla.mozilla.org/show_bug.cgi?id=1469592) of soon to be standardised [Element.toggleAttribute](https://github.com/whatwg/dom/pull/656). [Other implementers agreed to ship](https://github.com/whatwg/dom/issues/461)
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Allow supported window attributes to be replacable.
As seen in https://github.com/servo/servo/pull/21045 some Window attributes don't support JavaScript from being able to shadow and replace their values.
This PR adds all other attributes I could find that are supported by servo to be replaceable.
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add TextLength
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add TextLength in HTMLTextAreaElement
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Implement window.length DOM attribute.
Implements the `window.length` DOM attribute for enumerating frames on the document.
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Implement read methods on FileReaderSync
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Implemented the read methods on the FileReaderSync struct according to https://w3c.github.io/FileAPI/#dfn-FileReaderSync
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- [x] These changes fix#15114
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Implement MutationObserver.disconnect()
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This implements https://dom.spec.whatwg.org/#dom-mutationobserver-disconnect
I added a `node_list` to the `MutationObserver` struct to keep track of the nodes involved in an observer because otherwise I could not find a way to unregister the observers on the node themselves without traversing the whole document tree. Let me know if there's something I missed.
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Implement beforeunload event and infrastructure
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Implementation of:
1. https://html.spec.whatwg.org/multipage/#prompt-to-unload-a-document, and
2. https://html.spec.whatwg.org/multipage/#unload-a-document
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- [ ] These changes fix#10787 and fix#20485 and fix#20588 and fix#20496 (github issue number if applicable).
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MutationObserver api: Implement takeRecords() and characterData mutations
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I started by looking at adding support for `takeRecords()` to get fluent.js to work on Servo. To get the wpt tests to pass I ended up implementing character data mutations (most of the plumbing was already there) and this fixed a few more tests.
We are still missing support for `disconnect()`, which I keep for a followup.
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The test document_domain_setter.html now fails, but AFAIK the origin
comparison can never be true (document.origin is the unprefixed domain,
while the loaded iframe is from www1.…), and that test fails in all
major browsers because of security exceptions anyway.
TextDecoder's Decode now receives a BufferSource as input
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r? jdm
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