Async scrollbar dragging enabled on Nightly

[Botond Ballo]

Async scrolling (APZ) has been enabled for a new input method,
scrollbar dragging, on the Nightly branch.

This was initially implemented by Benoit Girard about a year ago,
briefly enabled on Nightly, and then disabled again due to regressions
that we didn't have the time to fix at the time. It languished on the
back burner until it was picked up recently by Kevin Wern, a volunteer
contributor who has been doing some great work for APZ. Thanks, Kevin,
for getting this in good enough shape to enable on Nightly again!

There are three known issues with this feature.

Two of them are hard to reproduce:
Bug 1249750 - Content area occasionally goes blank when dragging
scrollbar with APZ drag enabled
Bug 1250649 - async drag: Dragging while the scrollbar's max value
changes permanently breaks scrolling
If you encounter these, please post testcases / URLs to these bugs.

The third one, bug 1251617, is that a quick drag on a long page
results in checkerboarding. This is hard to fix, and can be thought of
as a conscious tradeoff: we're deliberately allowing you to drag
faster than we can paint the content you're moving over. If you have
any thoughts about this tradeoff, please post them to this bug.

If you encounter any other regressions related to scrollbar dragging,
please file bugs blocking bug 1211610, which is the tracking bug for
this work.

Thanks,
Botond

[Gervase Markham]

On 19/12/16 19:16, Botond Ballo wrote:
> The third one, bug 1251617, is that a quick drag on a long page
> results in checkerboarding.

Why do we paint a checkerboard rather than the default single background
colour of the page?

Checkerboarding makes it really clear Firefox can't keep up. The
background colour just makes it seem like the content hasn't rendered
yet, which seems less jarring as people see that all the time on page load.

Gerv

[Kartikaya Gupta]

On Wed, Dec 21, 2016 at 6:19 AM, Gervase Markham <ge...@mozilla.org> wrote:
> Why do we paint a checkerboard

We don't actually paint a checkerboard pattern.

> rather than the default single background
> colour of the page?

This is what we do. It's still *called* checkerboarding though. The
behaviour has changed, the name has not.

Cheers,
kats

[Botond Ballo]

On Mon, Dec 19, 2016 at 2:16 PM, Botond Ballo <bba...@mozilla.com> wrote:
> Async scrolling (APZ) has been enabled for a new input method,
> scrollbar dragging, on the Nightly branch.

Sorry, I should have mentioned this sooner, but this was backed out
due to a test failure before reaching m-c. So, any issues you might be
seeing with scrollbar dragging on currently m-c are not related to
this :)

I'll send another message when this lands and sticks.

Cheers,
Botond

[Botond Ballo]

> I'll send another message when this lands and sticks.

Async scrollbar dragging has now landed and stuck.

Cheers,
Botond