Correct overlay scrollbar check FrameView::scrollbarExistenceDidChange

Correct overlay scrollbar check FrameView::scrollbarExistenceDidChange

This issue is caused by FrameView::scrollbarExistenceDidChange calculate
usesOverlayScrollbars via check ScrollbarTheme::theme() only but ignore custom
scrollbar then pass layout.

In this patch, we add the custom scrollbar check to correct
usesOverlayScrollbars.

BUG=668387

[chaopeng, 2017-03-09 17:12:21]

chaopeng@chromium.org changed reviewers:
+ bokan@chromium.org

[chaopeng, 2017-03-09 17:12:21]

PTAL. Thank you.

Because we have a lot protected layout call, I can't build a simple case to test
this issue. Can you give me some advice? bokan@

Also I checked all calls of ScrollbarTheme::theme().usesOverlayScrollbars(), it
seems here is the only one place I need to fix.

[chaopeng, 2017-03-09 17:12:28]

The CQ bit was checked by chaopeng@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-03-09 17:13:16]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-03-09 18:44:37]

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

[commit-bot: I haz the power, 2017-03-09 18:44:38]

Dry run: This issue passed the CQ dry run.

[bokan, 2017-03-09 18:57:58]

On 2017/03/09 17:12:21, chaopeng wrote:
> PTAL. Thank you.
> 
> Because we have a lot protected layout call, I can't build a simple case to
test
> this issue. Can you give me some advice? bokan@
> 
> Also I checked all calls of ScrollbarTheme::theme().usesOverlayScrollbars(),
it
> seems here is the only one place I need to fix.

Code change looks good.

Re: test, one thing you could do is with a debugger or using StackTrace, find
out where the last couple of calls to this function are coming from. You want to
know how they change the scrollbar state but don't cause a layout otherwise.
This might give you a clue as to how to do that yourself.

You could also try loading a page with custom scrollbars but no overflow,
performing layout and all lifecycle updates, then adding some overflow that
doesn't cause layout. I believe transforming an element shouldn't cause a
layout. You could attach a debugger and step through to see why it is or isn't
hitting the code you expect. See the test I added in
https://codereview.chromium.org/2713773003/ which does something very similar.

[chaopeng, 2017-03-29 13:42:53]

Description was changed from

==========
Correct overlay scrollbar check FrameView::scrollbarExistenceDidChange

This issue is caused by FrameView::scrollbarExistenceDidChange calculate
usesOverlayScrollbars via check ScrollbarTheme::theme() only but ignore custom
scrollbar then pass layout.

In this patch, we add the custom scrollbar check to correct
usesOverlayScrollbars.

BUG=668387
==========

to

==========
Correct overlay scrollbar check FrameView::scrollbarExistenceDidChange

This issue is caused by FrameView::scrollbarExistenceDidChange calculate
usesOverlayScrollbars via check ScrollbarTheme::theme() only but ignore custom
scrollbar then pass layout.

In this patch, we add the custom scrollbar check to correct
usesOverlayScrollbars.

BUG=668387
==========