We need to ensure that we never promote low-DPI scrollers due to being opaque
that have non-integer offsets from the root cc::Layer. More generally it should
never happen that cc boots any layer out of LCD text moe if Blink promoted it because
it should be able to retain LCD text.

Marking this bug as blocking M55 stable.

Also we need to ensure that the screen space opacity is 1 (i.e. that no ancestor reduces our opacity): https://cs.chromium.org/chromium/src/cc/layers/layer_impl.cc?q=file:cc+opaque+lcd&sq=package:chromium&dr=CSs&l=1011

I put up a CL to make automatic promotion experimental again to fix this in M55: https://chromiumcodereview.appspot.com/2439573002.

As for M56 The scroll position should be pixel snapped by TransformTree::UpdateSnapping (https://cs.chromium.org/chromium/src/cc/trees/property_tree.cc?dr=CSs&q=file:cc%5C/+snap&sq=package:chromium&l=642), but we do need to check all ancestor transforms and opacity as per the check in #2.

Fix to not promote when we have transforms or opacity: https://chromiumcodereview.appspot.com/2434443005

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c656b155c2f6fb63288a259d130dd06dca393925

commit c656b155c2f6fb63288a259d130dd06dca393925
Author: flackr <flackr@chromium.org>
Date: Thu Oct 20 03:29:51 2016

Only automatically promote scrollers which are untransformed and opaque.

To be sure that we actually get LCD text, we only promote scrollers which have
not been transformed themselves or by any ancestor, and also exclude those
which are translucent due to an opacity on themselves or an ancestor.

BUG= 644833 
TEST=PaintLayerScrollableAreaTest.OnlyNonTransformedOpaqueLayersPromoted, PaintLayerScrollableAreaTest.OnlyOpaqueLayersPromoted
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://chromiumcodereview.appspot.com/2434443005
Cr-Commit-Position: refs/heads/master@{#426385}

[modify] https://crrev.com/c656b155c2f6fb63288a259d130dd06dca393925/third_party/WebKit/LayoutTests/compositing/overflow/scaled-overflow.html
[modify] https://crrev.com/c656b155c2f6fb63288a259d130dd06dca393925/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
[modify] https://crrev.com/c656b155c2f6fb63288a259d130dd06dca393925/third_party/WebKit/Source/core/paint/PaintLayerScrollableAreaTest.cpp
[modify] https://crrev.com/c656b155c2f6fb63288a259d130dd06dca393925/third_party/WebKit/Source/platform/testing/RuntimeEnabledFeaturesTestHelpers.h

Is this issue is fixed? If yes, Could any one let us know is there manual repro steps available to verify this issue from Chrome-TE end.

Thanks!

This issue is fixed, but we would like to disable the CompositeOpaqueScrollers feature in M-55 due to  issue 651305  as well. I just landed https://codereview.chromium.org/2439573002 to do this which I'd like to merge back - just waiting to see it show up in the bug here.

Hmm the bot that posts to the bugs must be really delayed, anyways I would like to merge https://chromium.googlesource.com/chromium/src/+/42f45334458475b43440671e6e7de5ff95e54a7c to M55 to avoid this issue on this bug and  issue 651305 .

Your change meets the bar and is auto-approved for M55 (branch: 2883)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c8338128808d2dae2ffc77a87a47554eeb7d4210

commit c8338128808d2dae2ffc77a87a47554eeb7d4210
Author: Robert Flack <flackr@chromium.org>
Date: Sat Oct 22 02:32:15 2016

Make CompositeOpaqueScrollers experimental until  https://crbug.com/644833  is fixed.

BUG= 644833 , 651305 

Review-Url: https://chromiumcodereview.appspot.com/2439573002
Cr-Commit-Position: refs/heads/master@{#426486}
(cherry picked from commit 42f45334458475b43440671e6e7de5ff95e54a7c)

Review URL: https://codereview.chromium.org/2444663002 .

Cr-Commit-Position: refs/branch-heads/2883@{#235}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}

[modify] https://crrev.com/c8338128808d2dae2ffc77a87a47554eeb7d4210/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/10336a25975c113e068268d848b01f0c3563f215

commit 10336a25975c113e068268d848b01f0c3563f215
Author: flackr <flackr@chromium.org>
Date: Mon Oct 24 20:27:38 2016

Reenable CompositeOpaqueScrollers on tip of tree.

BUG= 644833 , 651305 

Review-Url: https://codereview.chromium.org/2450473002
Cr-Commit-Position: refs/heads/master@{#427136}

[modify] https://crrev.com/10336a25975c113e068268d848b01f0c3563f215/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

This should be fixed on M56 and CompositeOpaqueScrollers is not on by default in M55. You can verify by visiting a page like http://output.jsbin.com/wacasinozi on a low dpi device and seeing that the first scroller still has subpixel text anti-aliasing. If you open dev tools and enable layer borders you can see that it is not promoted because it has a transform which may cause it to lose subpixel AA.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c8338128808d2dae2ffc77a87a47554eeb7d4210

commit c8338128808d2dae2ffc77a87a47554eeb7d4210
Author: Robert Flack <flackr@chromium.org>
Date: Sat Oct 22 02:32:15 2016

Make CompositeOpaqueScrollers experimental until  https://crbug.com/644833  is fixed.

BUG= 644833 , 651305 

Review-Url: https://chromiumcodereview.appspot.com/2439573002
Cr-Commit-Position: refs/heads/master@{#426486}
(cherry picked from commit 42f45334458475b43440671e6e7de5ff95e54a7c)

Review URL: https://codereview.chromium.org/2444663002 .

Cr-Commit-Position: refs/branch-heads/2883@{#235}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}

[modify] https://crrev.com/c8338128808d2dae2ffc77a87a47554eeb7d4210/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

It's still a bug in decimal DSF:
https://bugs.chromium.org/p/chromium/issues/detail?id=612270&can=1&q=dropdown%20border&colspec=ID%20Pri%20M%20Stars%20ReleaseBlock%20Component%20Status%20Owner%20Summary%20OS%20Modified

[Automated comment] removing mislabelled merge-merged-2840

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/318d3c7d33df6e1c6a16d929a57d350f77acfd91

commit 318d3c7d33df6e1c6a16d929a57d350f77acfd91
Author: Eric Willigers <ericwilligers@chromium.org>
Date: Wed Aug 08 17:22:16 2018

Retire CompositeOpaqueScrollers flag

CompositeOpaqueScrollers has been shipping since M56.
https://codereview.chromium.org/2450473002

BUG= 644833 ,  651305 

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2efd8b92d96177ebfa45b360c02a5cf5e31a18dc
Reviewed-on: https://chromium-review.googlesource.com/1166611
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581610}
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-background-opaque-to-transparent.html
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-background-transparent-to-opaque.html
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/WebKit/LayoutTests/compositing/overflow/overflow-scroll-with-opaque-background.html
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/blink/renderer/core/paint/compositing/compositing_reason_finder_test.cc
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/blink/renderer/core/paint/paint_layer_scrollable_area.cc
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/blink/renderer/core/paint/paint_layer_scrollable_area_test.cc
[modify] https://crrev.com/318d3c7d33df6e1c6a16d929a57d350f77acfd91/third_party/blink/renderer/platform/runtime_enabled_features.json5