1663612 - Ensure APZ handles scroll snapping with pure-relative scrolls

Status: NEW

(Core :: Panning and Zooming, task)

Description

[Kartikaya Gupta (email:[e-mail])]

https://hg.mozilla.org/integration/autoland/rev/51c3ae02e5ebe85771ca162de5ea2a54af15ba9a adds a check and bails out of the pure-relative codepath if scroll snapping is required. I think we shouldn't need to do this, and instead we should ensure that APZ respects scroll snapping on pure-relative requests from the main thread (and any other scroll requests).

Comment 1

[Kartikaya Gupta (email:[e-mail])]

Attachment: Test page with snapping divs

It's not really clear to me what needs to be done here. I played around a little with scroll snapping in combination with desktop zoom (page attached) and it seems somewhat buggy in general. e.g. immediately after zooming in we'll do a snap which is disconcerting. Often while zoomed in we'll snap to the left edge of the page even though there's only y-snapping in the CSS. Chrome doesn't seem to do either of those things.

Also I'm not sure if it's the visual viewport that should be snapping, or the layout viewport. I had imagined it would be the layout viewport, and the visual viewport would then allow you to zoom in and pan around to unsnapped coordinates, but maybe that's not the right model. Chrome seems to snap the visual viewport when zoomed in on the attached page.

Comment 2

[Kartikaya Gupta (email:[e-mail])]

I filed bug 1677199 for the more general problem with snapping. I'll unassign this for now since we probably need that resolved before we can figure out what needs to be done here.