Markus Stange [:mstange] Reporter
	Description • 9 years ago

Steps to reproduce:
 0. Turn on apz.minimap.enabled.
 1. Go to http://hpmor.com/chapter/6 and scroll to the middle of the page.
 2. On your Macbook Pro touchpad, alternatingly scroll up and down, lifting your
    fingers from the touchpad every time you change the scroll direction.
    Basically a series of small flings.
 3. Observe the display port position in the minimap.

The displayport sometimes randomly jumps to the top or the bottom of the page, and in general looks very twitchy.

Setting apz.velocity_bias to zero is a workaround that works.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 1 • 9 years ago

This is basically a more general description of the specific bug 1235906 that Jeff filed yesterday. I'll dupe that over.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 3 • 8 years ago

Once the telemetry is in we can land a patch to drop the velocity bias to 0 and see how it affects things.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 5 • 8 years ago

So I dug into this a bit more and the first move event definitely can generate some very crazy velocities. I expanded the logging in Axis::UpdateWithTouchAtDevicePoint to also log the distance and time deltas that result in the |newVelocity| computation and I see that sometimes the time delta is 1ms (which is what results in the giant velocities) and in fact sometimes it is -1ms. Individual move events tend to be a more reasonable 16-17ms apart. Fixing the velocity computation should fix this bug, I think.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 6 • 8 years ago

Can you try this patch and see if you still encounter the problem? I don't see it any more with this patch applied. (Note you'll want to set velocity bias back to 1.0 to make sure).

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 7 • 8 years ago

Comment on attachment 8713373 [details] [diff] [review]
Patch

Markus is going on PTO. Jeff, can you apply this patch and see if it fixes your velocity bias problems?

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 8 • 8 years ago

Here's a more cleaned-up patch that can actually be landed.

	Pulsebot
	Comment 9 • 8 years ago

https://hg.mozilla.org/integration/mozilla-inbound/rev/17a70b41806c

	Carsten Book [:Tomcat]
	Comment 10 • 8 years ago
bugherder

https://hg.mozilla.org/mozilla-central/rev/17a70b41806c

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 11 • 8 years ago

This is in the Feb05 nightly onwards, so we should check telemetry after a few days to see if it made a difference in checkerboarding.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 12 • 8 years ago

Based on the telemetry data I can't see a visible improvement. However it also didn't make anything worse, and while scrolling around on pages I see the displayport jump erratically much less, so I think this is worth uplifting anyway.

	Kartikaya Gupta (email:kats@mozilla.staktrace.com) Assignee
	Comment 13 • 8 years ago

Comment on attachment 8715316 [details] [diff] [review]
Patch (cleaned up)

Approval Request Comment
[Feature/regressing bug #]: APZ
[User impact if declined]: When starting a scroll the displayport can jump erratically, causing checkerboarding. This patch fixes the velocity computation which fixes the erratic jumps, and should reduce instances of checkerboarding.
[Describe test coverage new/current, TreeHerder]: locally, on m-c for a while now
[Risks and why]: low risk, it just avoids sampling velocity over very small timespans. Code is well understood.
[String/UUID change made/needed]: none

	Liz Henry (:lizzard Please n-i to RyanVM, jcristau, or pascal)
	Comment 14 • 8 years ago

Comment on attachment 8715316 [details] [diff] [review]
Patch (cleaned up)

Sounds like this may avoid some apz scrolling issues, please uplift to aurora.

	Carsten Book [:Tomcat]
	Comment 15 • 8 years ago
bugherder uplift

https://hg.mozilla.org/releases/mozilla-aurora/rev/e9d96b7d7d7b