Does it really? What about overflow:scroll? Paint invalidation would be wrong then if it clipped paint invalidation rects inside of a composited scroller, right?

Look at the result for fast/clip/014.html in https://storage.googleapis.com/chromium-layout-test-archives/linux_chromium_rel_ng/201167/layout-test-results/results.html

Seems like a related problem, as we apparently don't clip the scrollbars on the now-composited content. Could be totally unrelated, of course.

Re comment 2: there are some "corner-case" bugs in composited clipping, this could be one of those. I think it is likely not the same issue.

To clarify: the bug applies to the slow path only. This will apply to the whole LayoutObject::mapToVisibleRectInAncestorSpace() after my change which moves fast-path mapping out of the function.

This also applies to non-composited-scrolling ancestors only. We don't need to clip in a composited-scrolling ancestor.

We decided not to apply the outer-most clip in mapToVisualRectInAncestorSpace().

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2

commit ad7b21ace472d41b5b90ddab45f5e7bd44e737f2
Author: wangxianzhu <wangxianzhu@chromium.org>
Date: Thu Apr 07 23:25:22 2016

Let mapToVisualRectInAncestorSpace apply ancestor clip if not scrollsOverflow

BUG= 597902 

Review URL: https://codereview.chromium.org/1856993004

Cr-Commit-Position: refs/heads/master@{#385919}

[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/LayoutBox.cpp
[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/LayoutBox.h
[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/LayoutInline.cpp
[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/LayoutObject.cpp
[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/LayoutObject.h
[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
[modify] https://crrev.com/ad7b21ace472d41b5b90ddab45f5e7bd44e737f2/third_party/WebKit/Source/core/layout/VisualRectMappingTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/537724421c0f5d046e56fb0eba001da3bcea0613

commit 537724421c0f5d046e56fb0eba001da3bcea0613
Author: chrishtr <chrishtr@chromium.org>
Date: Sun Aug 28 02:15:49 2016

Don't apply overflow clips in paint invalidation.

Previously, such clips were applied if the paint invalidation container did
not scroll overflow, but this is problematic, because overflow:hidden elements
can be scrolled via script.

BUG= 597902 , 640565 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2280243002
Cr-Commit-Position: refs/heads/master@{#414951}

[modify] https://crrev.com/537724421c0f5d046e56fb0eba001da3bcea0613/third_party/WebKit/LayoutTests/TestExpectations
[add] https://crrev.com/537724421c0f5d046e56fb0eba001da3bcea0613/third_party/WebKit/LayoutTests/paint/invalidation/overflow-hidden-yet-scrolled-expected.txt
[add] https://crrev.com/537724421c0f5d046e56fb0eba001da3bcea0613/third_party/WebKit/LayoutTests/paint/invalidation/overflow-hidden-yet-scrolled.html
[modify] https://crrev.com/537724421c0f5d046e56fb0eba001da3bcea0613/third_party/WebKit/Source/core/layout/LayoutBox.cpp
[modify] https://crrev.com/537724421c0f5d046e56fb0eba001da3bcea0613/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
[modify] https://crrev.com/537724421c0f5d046e56fb0eba001da3bcea0613/third_party/WebKit/Source/core/layout/VisualRectMappingTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/19e275edf518255cc77cf7fe97d501f536ec87b7

commit 19e275edf518255cc77cf7fe97d501f536ec87b7
Author: Chris Harrelson <chrishtr@chromium.org>
Date: Mon Aug 29 21:34:20 2016

Don't apply overflow clips in paint invalidation.

Previously, such clips were applied if the paint invalidation container did
not scroll overflow, but this is problematic, because overflow:hidden elements
can be scrolled via script.

BUG= 597902 , 640565 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2280243002
Cr-Commit-Position: refs/heads/master@{#414951}
(cherry picked from commit 537724421c0f5d046e56fb0eba001da3bcea0613)

Review URL: https://codereview.chromium.org/2295503002 .

Cr-Commit-Position: refs/branch-heads/2840@{#24}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/LayoutTests/TestExpectations
[add] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/LayoutTests/paint/invalidation/overflow-hidden-yet-scrolled-expected.txt
[add] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/LayoutTests/paint/invalidation/overflow-hidden-yet-scrolled.html
[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/Source/core/layout/LayoutBox.cpp
[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/Source/core/layout/VisualRectMappingTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/19e275edf518255cc77cf7fe97d501f536ec87b7

commit 19e275edf518255cc77cf7fe97d501f536ec87b7
Author: Chris Harrelson <chrishtr@chromium.org>
Date: Mon Aug 29 21:34:20 2016

Don't apply overflow clips in paint invalidation.

Previously, such clips were applied if the paint invalidation container did
not scroll overflow, but this is problematic, because overflow:hidden elements
can be scrolled via script.

BUG= 597902 , 640565 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2280243002
Cr-Commit-Position: refs/heads/master@{#414951}
(cherry picked from commit 537724421c0f5d046e56fb0eba001da3bcea0613)

Review URL: https://codereview.chromium.org/2295503002 .

Cr-Commit-Position: refs/branch-heads/2840@{#24}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/LayoutTests/TestExpectations
[add] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/LayoutTests/paint/invalidation/overflow-hidden-yet-scrolled-expected.txt
[add] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/LayoutTests/paint/invalidation/overflow-hidden-yet-scrolled.html
[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/Source/core/layout/LayoutBox.cpp
[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/Source/core/layout/PaintInvalidationState.cpp
[modify] https://crrev.com/19e275edf518255cc77cf7fe97d501f536ec87b7/third_party/WebKit/Source/core/layout/VisualRectMappingTest.cpp