[cssom-view] Fire scrollend consistently in cases where there is no scrolling.

[flackr]

While experimenting with the implementations of scrollend in firefox and chrome I noticed that there's some inconsistency about in which cases it fires when no scrolling occurs. In particular, scrollend is fired when the user pressed an arrow key, or performs a scrolling action which results in no scroll, but is not fired when a programmatic API is used to scroll to the current location.

While I can see an ergonomic argument for strictly pairing scrollend to scroll events, I think it would be useful if we guaranteed a scrollend event will occur after scrolling API calls, enabling developers to do something after a scroll to a location without needing to detect if scroll didn't happen. e.g.

function scrollToPromise(element) {
  return new Promise((resolve) => {
    window.addEventListener('scrollend', resolve);
    element.scrollIntoViewIfNeeded();
  });
}

@argyleink

[flackr]

Though if we make the scrolling APIs (scrollIntoView, scrollIntoViewIfNeeded, scrollTo, etc) return promises which resolve when the scroll is complete I could see never firing scrollend if there is no scroll.

I'm not sure whether the current spec text is clear on what the behavior should be to the current text,

Whenever scrolling is completed, the user agent must run these steps

This could be interpreted either way I think.

[flackr]

In fact, on further reflection I strongly prefer scrollend strongly paired to scroll events, and developers relying on promises for scroll apis. So TLDR:

1. Update / ensure definition for when scrolling is completed used in cssom-view-1 spec Scrolling section does not occur if there was no scroll.
2. Return promises from scrolling APIs. Seems to have been resolved in issue [cssom-view] Consider making scroll methods return promises #1562 but not done yet.

[argyleink]

I feel like keeping scrollend "simple" in that it fires after scroll when there's no more translations or user input happening. if the scroll is requested to a new location, but it happens its the current location, there was no scroll so no scrollend event. It feels much more appropriate for the scrollTo functions to provide a callback or promise that the request was fulfilled, regardless of whether scroll actually occurred.

the problem space of this issue seems to be "it's hard to know that no scrolling occurred after programmatic assignment" and I think that problem lives at a higher level than scrollend, aka it's not scrollend's responsibility to remedy it. i do agree this is a real issue though and scroll api's should offer ways to reduce this mystery.

[argyleink]

ensure definition

what kind of updates would reduce ambiguity for you? have suggestions 🙂

[flackr]

So there is a note that calls this out that no scrolling produces no scrollend, but if I read the normative text from https://drafts.csswg.org/cssom-view/#scrolling it sounds as if scrollend is always emitted.

I.e. taking the instant case (because it's simpler), the following steps must be run:

1. Abort any ongoing smooth scroll for box.
2. perform an instant scroll of box to position. After an instant scroll emit the scrollend event.

This needs normative text to say something like if the scroll position was not originally "position" then emit a scrollend event.

[argyleink]

I just tested out adding some of this additional copy,and it sounds repetitive while also having the note directly following the new copy about not emitting the event if no translation occurred. I'm worried it'll clutter and repeat the copy instead of make it clearer. The spec starts to sound really concerned you'll not see the note. Thoughts? maybe adding more tests for these cases is better than copy edits?

[argyleink]

this still relevant you feel or covered via updates since this issue was created?

[flackr]

I still think the normative text is wrong. What if we add a step before step 2 after the abort in step 1 that says if the box is already scrolled to position, abort the remaining steps?

This would be similar to step 3 in the procedure to scroll to the beginning of the document

[dlrobertson]

This seems like this would be solved by #8397