This patch basically attempts to make clicking in the scrollbar track outside of the scrollthumb "work". Clicking in the scrollbar track usually does a page scroll via nsSliderFrame::PageScroll. This eventually ends up in ScrollFrameHelper::ScrollBy where we turn the request from a relative one ("scroll by a page") into an absolute one ("scroll to this position").
This page scroll is typically a smooth scroll and is currently done on the main thread/layout side. Once we start scrolling the visual viewport offset with the scrollbars we can no longer do this purely on the layout side, we at least need the help of the compositor side. I think the simplest way to do this is to hand the scroll request off to the compositor and have it handle the whole thing.
Now we need to consider the following situation: user clicks scrollbar track to page scroll, smooth scroll gets partway complete on the compositor, user clicks again on scrollbar track for a further page scroll. The main thread can't send an absolute scroll offset update request to the compositor at this point because it has outdated information (it needs a 'starting position' to add the page scroll offset amount) so it'll end up scrolling to the wrong place. It has to send a relative scroll offset update.
We already have a mechanism to send relative scroll offset updates. It is implemented by sending a base scroll offset and the desired scroll offset, and then on the compositor send the difference between those two is computed and then added to the scroll offset.
This patch creates a new mechanism (so called "pure relative") that just sends a relative offset update without any absolute scroll positions. The reason I did this is because the existing relative scroll offset update mechanism is not aware of visual viewport offsets, but rather only layout scroll position. For example, here
https://searchfox.org/mozilla-central/rev/8d55e18875b89cdf2a22a7cba60dc40999c18356/layout/generic/nsGfxScrollFrame.h#446
the value we use for the base scroll offset (mApzScrollPos) is set to the layout scroll position. It may be entirely reasonable to make this existing mechanism vv offset aware, but I wanted to implement something to get it working with a smaller chance of regressions to things that already exist and work. Ideally these two mechanims would be merged.
Differential Revision: https://phabricator.services.mozilla.com/D82688
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