Feat(#32): add Element.scroll/To/By methods

[ghybs]

as per new draft spec version, as mentioned in issue #32
https://drafts.csswg.org/cssom-view/#extension-to-the-element-interface

This PR only adds Element.prototype.scrollBy, it does not address scrollTo nor anything else yet.

Also added a dedicated section on the index.html demo page for immediate test:
https://ghybs.github.io/smoothscroll/

Tested on Firefox 51, Chromium 53, Opera 43.

NOTE: it might be interesting to refactor Element.prototype.scrollIntoView call to smoothScroll using scrollableParent.scrollBy?

[ghybs]

Now added Element.prototype.scroll and Element.prototype.scrollTo, which enabled fixing the fallback (bailout) of Element.prototype.scrollBy.

This should address the entire feature suggestion of #32.

[ghybs]

Tested on:

• Firefox 51
• Firefox for Android 51
• Firefox for iOS 6
• Chromium 53 & 56
• Chrome for Android 56
• Chrome for iOS 56
• Opera 43
• Safari 10
• iOS Safari 10

[iamdustan]

This is great. Thanks @ghybs. One minor change and it looks good to me!

[iamdustan]

We shouldn’t mutate incoming arguments. We’ll likely want to introduce a really small and simple assign method to handle this.

this.scroll(assign({}, arg0, {
  left: this.scrollLeft + arg0.left,
  top: this.scrollTop + arg0.top,
  behavior: arg0.behavior,
}));

[ghybs]

Nice thought about changed input argument.
Here you go.

I did not implement a full assign function (I guess you wanted it to clone the input?), as it looked quite overkill for just a simple use with 3 object members (always the same keys) to take care of.
Please let me know if I missed something.

[iamdustan]

👍 If the spec changes and adds new features we’ll just need to update then.

Appreash!