1785918 - Programmatic smooth scrolls do not DispatchStateChangeNotification when they finish

Status: RESOLVED INVALID

(Core :: Panning and Zooming, defect)

Description

[Dan Robertson (:dlrobertson)]

Given the following in a mochitest, the DispatchStateChangeNotification will not be called when the smooth scroll finishes and the APZC will not change states from SMOOTHMSD_SCROLL to NOTHING:

scrollBy({ top: 2000, left: 0, behavior: "smooth" });

await promiseApzFlushedRepaints();
// The APZC will still be in the SMOOTHMSD_SCROLL state here

This may not cause issues currently, but will mean that when scrollend is supported no scrollend event will be posted when the programmatic scroll is completed.

Comment 1

[Hiroyuki Ikezoe (:hiro)]

To me, an await promiseApzFlushedRepaints() doesn't ensure its async smooth scroll end. Do you mean awaiting a promiseTransformEnd() promise?

Comment 2

[Dan Robertson (:dlrobertson)]

(In reply to Hiroyuki Ikezoe (:hiro) from comment #1)

To me, an await promiseApzFlushedRepaints() doesn't ensure its async smooth scroll end. Do you mean awaiting a promiseTransformEnd() promise?

Ah, you're right! For some reason I thought promiseApzFlushedRepaints would ensure the smooth scroll end.