Avoid div-by-zero in thumb quad computation

Avoid div-by-zero in thumb quad computation

We used to have trouble when the max scroll offset was zero. With this
cl we detect this case and set a reasonable scroll thumb offset.

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284773

[weiliangc, 2014-07-22 18:49:46]

https://codereview.chromium.org/409193002/diff/20001/cc/layers/scrollbar_laye...
File cc/layers/scrollbar_layer_unittest.cc (right):

https://codereview.chromium.org/409193002/diff/20001/cc/layers/scrollbar_laye...
cc/layers/scrollbar_layer_unittest.cc:277: const int kTrackStart = 1;
I don't understand why we need to change TrackStart constant in test?

[Ian Vollick, 2014-07-22 18:55:00]

https://codereview.chromium.org/409193002/diff/20001/cc/layers/scrollbar_laye...
File cc/layers/scrollbar_layer_unittest.cc (right):

https://codereview.chromium.org/409193002/diff/20001/cc/layers/scrollbar_laye...
cc/layers/scrollbar_layer_unittest.cc:277: const int kTrackStart = 1;
On 2014/07/22 18:49:46, weiliangc wrote:
> I don't understand why we need to change TrackStart constant in test?

It's a good question. I did this so that I can more reliably check that we set
the TrackStart value when we avoid the if block in scrollbar_layer_base_impl
when maximum_ is zero (rather than coincidentally winding up with zero if the
numerics fell apart).

I also think it's a nicer value than zero, even for the other tests -- if it's
non-zero, we can actually see it being used.

[weiliangc, 2014-07-22 19:00:13]

lgtm

https://codereview.chromium.org/409193002/diff/20001/cc/layers/scrollbar_laye...
File cc/layers/scrollbar_layer_unittest.cc (right):

https://codereview.chromium.org/409193002/diff/20001/cc/layers/scrollbar_laye...
cc/layers/scrollbar_layer_unittest.cc:277: const int kTrackStart = 1;
On 2014/07/22 18:55:00, Ian Vollick wrote:
> On 2014/07/22 18:49:46, weiliangc wrote:
> > I don't understand why we need to change TrackStart constant in test?
> 
> It's a good question. I did this so that I can more reliably check that we set
> the TrackStart value when we avoid the if block in scrollbar_layer_base_impl
> when maximum_ is zero (rather than coincidentally winding up with zero if the
> numerics fell apart).
> 
> I also think it's a nicer value than zero, even for the other tests -- if it's
> non-zero, we can actually see it being used.

Ah didn't realize the first test case has 40->39 and second test case 80->78 is
because it's 2x the first time, thought second case decremented more than first
test case because of rounding issue.

Yeah, looks good.

[Ian Vollick, 2014-07-22 19:01:32]

The CQ bit was checked by vollick@chromium.org

[commit-bot: I haz the power, 2014-07-22 19:03:24]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vollick@chromium.org/409193002/20001

[commit-bot: I haz the power, 2014-07-22 19:03:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-07-22 19:03:26]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[ajuma, 2014-07-22 19:09:44]

lgtm

[enne (OOO), 2014-07-22 19:11:26]

lgtm

[Ian Vollick, 2014-07-22 19:13:30]

The CQ bit was checked by vollick@chromium.org

[commit-bot: I haz the power, 2014-07-22 19:14:35]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/vollick@chromium.org/409193002/20001

[commit-bot: I haz the power, 2014-07-22 21:16:27]

Change committed as 284773