Support {block: 'end'}

[DanielHeath]

I've broken my work into four logically separate commits for easy review.

The test harness renders a sequence of iframes with different setups. I found it invaluable when comparing behavior across browsers.

Adding support for overriding browser builtins made comparative testing much easier. I also plan to use it in my application because firefox's implementation has bugs.

Renaming shouldBailOut to scrollIsInstant makes the block: end stuff read much better.

The final change actually implements the block: 'end' scroll behavior.

[DanielHeath]

This isn't quite right yet - there's an issue I'm still trying to track down - but I'd be surprised if the final implementation was very different.

[DanielHeath]

The recursive call to scrollIntoView isn't going to play nicely with scrolling to an element at the middle of a very long scrollable parent. Will have to rethink 71ac7f6 with that in mind.

[DanielHeath]

OK, figured it out.

I suspect this didn't (and still doesn't) work if you nest scrollable elements arbitrarily deep, but that's a terrible UX anyways and practically nobody does it.

[iamdustan]

oh dang. This looks solid, @DanielHeath. Sorry for neglecting it for so long. Would you be alright picking this back up after #54 lands? I’ll get you a proper review then if you’re game.

[DanielHeath]

I'm about to go on paternity leave (woot) so probably won't be about much.

There's a bug on chrome >= 56 that stops it cleaning up after itself properly - after scrolling to something it breaks further attempts to scroll. Haven't had time to investigate yet.

[iamdustan]

Oh awesome! Congratulations, @DanielHeath! Enjoy dadmode! 😄

[pmyagkov]

@iamdustan @DanielHeath guys, could you actualise this PR and merge it? It's a crucial feature for me to use.

UPD: I created a PR #88 based on the most recent master and this PR with { block: 'end' } support. Could you please merge it and bump a version?