fix: reset title element to previous value on removal

[dummdidumm]

This started as an attempt to fix #7656, but I now ran into some questions where I'm not sure what's the best answer, to I'm pushing this up as a WIP

The questions:

• is resetting the title to the previous value the correct move? or should we reset it to the empty string?
• assuming we reset it to the previous value, what does happen if you have multiple title elements at the same time in different components, and the parent title is set to A, then the child title is set to B, then the parent title updates its title to A2. Upon removal, with this PR, it would go back too A. Is that fine?

[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14116

commit: 3265021

[trueadm]

I wonder what other frameworks do here? However, it's maybe the correct behaviour if another <title> set the value.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14116-svelte.vercel.app/

this is an automated message

[Rich-Harris]

I haven't been able to break this, but it feels like we should put the setup in an effect so that there's no possibility of another component setting the title to a new value immediately before the teardown nukes it. might just be superstition

Suggested change

	const previous = document.title;
	document.title = text;
	teardown(() => {
	document.title = previous;
	});
	const previous = document.title;
	effect(() => {
	document.title = text;
	return () => {
	document.title = previous;
	};
	});

[dummdidumm]

The effect does nothing, since text is a static value; I kept the "add document.title = .. to the shared effect" part within the compiler which is why it's updating still. The assumption is that it's rare that people have a reactive title. But we can move it into here

[Rich-Harris]

It's not about reactivity, it's about preventing this sequence of events from being a possibility:

1. component A is removed and component B is created at the same moment
2. component B sets document.title
3. component A is removed and reverts document.title to whatever it was before A was created

That may already be impossible, hence 'superstition'

[dummdidumm]

What we could also do: Last one wins, always, i.e. it doesn't matter whether or not A is first and updates after B comes in, if B is after A then B always takes precedence. In DEV mode we could also warn then "hey there are multiple competing, and the last one which is currently X wins".

[trueadm]

What we could also do: Last one wins, always, i.e. it doesn't matter whether or not A is first and updates after B comes in, if B is after A then B always takes precedence. In DEV mode we could also warn then "hey there are multiple competing, and the last one which is currently X wins".

That gets complicated with error boundaries though, no? What if you trigger an update and the title changes, but then an error occurs? Is this warning then useful here?

[Rich-Harris]

That gets complicated with error boundaries though, no? What if you trigger an update and the title changes, but then an error occurs?

that feels like a good reason to put the assignment to document.title in an effect