[cssom-view] Should the associated element to window.scroll() be the scrollingElement?

[fred-wang]

The associated element is used to determine the scroll behavior when the one in ScrollOptions is auto ( https://drafts.csswg.org/cssom-view/#perform-a-scroll ). For window.scroll(), it is defined as the document’s root element ( https://drafts.csswg.org/cssom-view/#dom-window-scroll ):

Perform a scroll of the viewport to position, document’s root element as the associated element, if there is one, or null otherwise, and the scroll behavior being the value of the behavior dictionary member of options.

However, window.scroll() is also executed during Element.scroll() for two cases (steps 8 and 9), essentially when the function is invoked on the scrollingElement ( https://drafts.csswg.org/cssom-view/#dom-element-scroll ):

If the element is the root element invoke scroll() on window with scrollX on window as first argument and y as second argument, and terminate these steps.
If the element is the HTML body element, document is in quirks mode, and the element is not potentially scrollable, invoke scroll() on window with options as the only argument, and terminate these steps.

One could expect that the CSS scroll-behavior is taken from the element on which scroll() is called, which is not the true in the latter case. Should window.scroll() use the scrollingElement as an associated element instead?

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Note: Other places where document’s root element is used are:
https://drafts.csswg.org/cssom-view/#scroll-to-the-beginning-of-the-document
https://drafts.csswg.org/cssom-view/#scroll-an-element-into-view

[fred-wang]

cc' @mrego @javifernandez @smfr @RByers

WebKit bug for scroll behavior: https://bugs.webkit.org/show_bug.cgi?id=188043

[fred-wang]

cc @cathiechen @bkardell

@smfr comment on the webkit bug:

I would expect the scroll options of the body or document to be consulted, following how overflow is propagated from the body to the root.

https://bugs.webkit.org/show_bug.cgi?id=188043#c103

[fred-wang]

Checking the spec again:

The scroll-behavior property specifies the scrolling behavior for a scrolling box, when scrolling happens due to navigation or CSSOM scrolling APIs. Any other scrolls, e.g. those that are performed by the user, are not affected by this property. When this property is specified on the root element, it applies to the viewport instead.

The scroll-behavior property of the HTML body element is not propagated to the viewport.

So it seems the intention was really to use the documentElement (not the scrollingElement).