Add noscroll option to goto, document sapper-noscroll on links

[Jayphen]

This adds an option to goto - noscroll: boolean - which allows preservation of scroll position when navigating programmatically.

noscroll already exists as an option on navigate, and this just passes the option through to that underlying method.

I've added tests that show the default behaviour when using goto (scrolling to the top), and the preservation behaviour when the option is passed.

This implements #584

Note: In working on this, I discovered a bug in goto/navigate - the scroll position is not preserved when using noscroll: true when only the search params are updated (not the pathname). I will add a separate issue for this.

edit: added issue with test cases

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

• [-] It's really useful if your PR relates to an outstanding issue, so please reference it in your PR, or create an explanatory one for discussion. In many cases features are absent for a reason.
• [-] This message body should clearly illustrate what problems it solves. If there are related issues, remember to reference them.
• [-] Ideally, include a test that fails without this PR but passes with it. PRs will only be merged once they pass CI. (Remember to npm run lint!)

Tests

• [-] Run the tests tests with npm test or yarn test)

[antony]

@Jayphen does this supercede the other PR?

[Jayphen]

@antony Yes, the other can be closed. It did not actually implement the change correctly.

[benmccann]

Can you add this to the documentation as well? https://github.com/sveltejs/sapper/tree/master/site/content/docs

[Jayphen]

Sure, should I also document the currently undocumented noscroll attribute that can be used on anchors?

[benmccann]

Yes, that'd be great. Thanks!

[Jayphen]

@benmccann Okay, I've added some docs. I copied the style of the Svelte docs for describing the type and default for each option: https://svelte.dev/docs#flip

[benmccann]

thanks for this @Jayphen ! this is a really high quality contribution. appreciate the docs and tests

[antony]

Great PR!

[lukeed]

Personally I'd prefer noscroll attribute. Maybe others can weigh in?

Suggested change

	<a href='path' sapper-noscroll>Path</a>
	<a href='path' noscroll>Path</a>

[arxpoetica]

@lukeed agree

[pngwn]

stickywhenclicky imo

[lukeed]

😂

[Jayphen]

This change is not relevant to this PR — I only documented what currently exists

[antony]

I think I prefer sapper-noscroll which makes it clear that it's a Sapper specific thing. 'noscroll' feels like an built-in attribute.

[benmccann]

I took a poll of the maintainers offline and there was a preference for keeping noscroll namespaced. However, folks preferred sapper:noscroll to sapper-noscroll, so I updated this PR to rename the attribute.

I'll go ahead and merge this now. Thanks for the patience and input everyone!

[jhwheeler]

I also prefer sapper:noscroll, but I'm curious, are there other namespaced attributes for html elements?

[Conduitry]

Not currently, but that's where things are going to be moving in the future. rel="prefetch" (which, when used on an anchor element, is a Sapper-only thing, which is confusingly named the same as a real HTML thing) is going to be renamed to some other namespaced sapper: attribute.

[jhwheeler]

Great, I like this idea. Much clearer!