Suppress scrollbar animation if the scroll gesture does not scroll

Suppress scrollbar animation if the scroll gesture does not scroll

Currently, the scrollbar layer animation is triggered at the end of every scroll
gesture. However, as not every scroll gesture actually scrolls a layer, this may
trigger extraneous animations, e.g., when the user overscrolls, or presses and
lifts their finger without scrolling.  Only trigger the scrollbar animation at
the end of a scroll gesture if the gesture caused the layer to scroll.

BUG=285771
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=221801

[jdduke (slow), 2013-09-05 19:01:29]

aelias@: PTAL.  If you think this is the right general direction, I'll update
the unit tests.

This makes the scrollbar animation trigger comparable with the Android system
scrollbar animation, which only activates if the layer scrolls.

[jdduke (slow), 2013-09-05 19:05:12]

I should also mention that this introduces a slight difference from current
behavior; the animation will not be activated by movement of only the top
controls.

[tony, 2013-09-05 19:16:42]

Is this really the desired behavior?  It seems like a nice visual cue to have
the scrollbar appear when at a scrolling edge.  It helps explain why the page
didn't scroll.

[jdduke (slow), 2013-09-05 19:26:04]

On 2013/09/05 19:16:42, tony wrote:
> Is this really the desired behavior?  It seems like a nice visual cue to have
> the scrollbar appear when at a scrolling edge.  It helps explain why the page
> didn't scroll.

That's why we have the overscroll effect.

[jdduke (slow), 2013-09-05 19:26:59]

On 2013/09/05 19:26:04, jdduke wrote:
> On 2013/09/05 19:16:42, tony wrote:
> > Is this really the desired behavior?  It seems like a nice visual cue to
have
> > the scrollbar appear when at a scrolling edge.  It helps explain why the
page
> > didn't scroll.
> 
> That's why we have the overscroll effect.

As it stands, if you're at the top of the screen, and you pull down, the
scrollbar won't appear until you release the pull, so it's not really an active
cue in that sense.

[jdduke (slow), 2013-09-05 20:21:47]

Hmm, then again I can see the value in showing the scrollbar even if we don't
scroll, I think we just need to be consistent.  As it stands, if you overscroll
when you're at an edge, the scrollbar won't appear until after you release your
finger, which is a somewhat surprising result.  Perhaps it's just a bug in the
opacity layer update?

[aelias_OOO_until_Jul13, 2013-09-05 22:27:41]

This approach looks fine.  You can go ahead and update the unit tests (and add
another one for this particular case).

On 2013/09/05 19:16:42, tony wrote:
> Is this really the desired behavior?  It seems like a nice visual cue to have
> the scrollbar appear when at a scrolling edge.  It helps explain why the page
> didn't scroll.

If I look at any other Android app using system scrollbars, they don't show up
when you overscroll.  So yes, this is the desired behavior.

[jdduke (slow), 2013-09-06 16:33:44]

Tests updated.

[aelias_OOO_until_Jul13, 2013-09-06 18:27:20]

lgtm

[commit-bot: I haz the power, 2013-09-06 18:47:45]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jdduke@chromium.org/23978008/18001

[commit-bot: I haz the power, 2013-09-06 22:33:52]

Change committed as 221801