fix: add logic to select and claim raw html tags in head

[myisaak]

This PR fixes #6463 with duplicated tags in head (and possibly #6832). Thanks to the helpful investigations from @hbirler #6463 (comment) and from @rmunn #6832 (comment), the problem had to do with the selector logic for claimable elements inside head, that only selected tags with svelte-data prop. I came up with a solution that extended it further to include raw html tags that are between HTML_TAG_START and HTML_TAG_END comments.

Note that for this logic I've added an internal api method called get_hydratable_raw_elements:

export function get_hydratable_raw_elements(parent: HTMLElement = document.body) {
	const elements = Array.from(parent.childNodes);
	let cursor = 0;
	while (cursor < elements.length) {
		const start_index = find_comment(elements, 'HTML_TAG_START', cursor);
		const end_index = find_comment(elements, 'HTML_TAG_END', start_index);
		if (start_index === end_index) {
			elements.splice(cursor, elements.length - cursor);
			break;
		}

		detach(elements[start_index]);
		detach(elements[end_index]);

		elements.splice(cursor, 1 + start_index - cursor);
		cursor += end_index - start_index - 1;
	}
	return elements;
}

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[myisaak]

Ok, I'm assuming the PR needs more validation, since 2 weeks have passed and it's still unmerged. Therefore I'll write a test and include it in this PR.

[tanhauhau]

what happends if you have multiple {@html 'xxx'} in the head?

how woudl you ensure you are not claiming the html tag of other components?

[tanhauhau]

and yes, please include a test case for this.

[myisaak]

I had some trouble writing tests that pass when checking for tags in head. Could you perhaps point me to some documentation? I couldn't find any do far.

[benmccann]

#6463 was fixed by #7745. Is this PR still needed?

[dummdidumm]

You're right, this should no longer be needed - closing, thank you.