I put the demo into jsbin:

https://jsbin.com/zemipipife/edit?html,output

Stefan, are you actually thinking of working on this?

No, I'm not :)  Aleks gave it to me because I told him to file the bug.

Unassigning.

trchen said he'd be interested in looking into this in LegacyLayout.

chrishtr@: Another spec bug! Not urgent, but is something we need to think about.

It seems to me that the transformed element's post-transform box has been included in layout overflow, but is computed incorrectly. Let's say if we changed it to transform:translateY(100px)rotateY(-45deg), the box will be scrollable by 100px.

I think it is a bug with PropertyTreeBuilder/GeometryMapper that we failed to include perspective in layout overflow computation (but correctly included it during paint/draw).

This is a tricky case though. The transform spec wasn't so clear whether perspective should be applied before or after flattening, and whether perspective origin follows scrolling if applied before flattening.

Web developers prefer to apply perspective before, and also prefer perspective origin anchor to the (unscrolled) border box, in order to support parallax. But I didn't realize if we do this, the transformed layout overflow will depend on scroll offset. We need to think about what's the reasonable thing to do here and propose it to the spec. :/

PropertyTreeBuilder/GeometryMapper are not currently used to compute layout overflow.
The logic appears to be in LayoutBox::LayoutOverflowRectForPropagation(), which
does this:

LayoutRect rect = BorderBoxRect();
Layer()->CurrentTransform().MapRect(rect);

Furthermore, only children whose containing block is the scroller are included (of course)
Therefore, we should use LayoutObject::GetTransformFromContainer, which will apply perspective.

As for flattening, we decided that that is applied after perspective. See:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutBox.cpp?sq=package:chromium&dr=CSs&l=1219

in LayoutBox::MapVisualRectToContainer.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3

commit 602f4cdf9e0944a68fe91d12ead5b2ece186b8e3
Author: Chris Harrelson <chrishtr@chromium.org>
Date: Sat Jun 24 01:00:35 2017

Account for perspective of the scroller when determining a transformed
child's contribution to layout overflow.

Bug:  723788 
Change-Id: I18902203e16b058d3f2fa715c10605ebf8a50d36
Reviewed-on: https://chromium-review.googlesource.com/539059
Commit-Queue: Chris harrelson <chrishtr@chromium.org>
Reviewed-by: Tien-Ren Chen <trchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482101}
[modify] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/compositing/overflow/accelerated-overflow-scroll-should-not-affect-perspective-expected.txt
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/overflow/overflow-transform-perspective.html
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/linux/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/linux/overflow/overflow-transform-perspective-expected.txt
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/linux/virtual/layout_ng/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/linux/virtual/layout_ng/overflow/overflow-transform-perspective-expected.txt
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/mac-mac10.9/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/mac-mac10.9/virtual/layout_ng/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/mac/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/mac/overflow/overflow-transform-perspective-expected.txt
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/mac/virtual/layout_ng/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/mac/virtual/layout_ng/overflow/overflow-transform-perspective-expected.txt
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/win/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/win/overflow/overflow-transform-perspective-expected.txt
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/win/virtual/layout_ng/overflow/overflow-transform-perspective-expected.png
[add] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/LayoutTests/platform/win/virtual/layout_ng/overflow/overflow-transform-perspective-expected.txt
[delete] https://crrev.com/adefd5760bfb4138b462801d4c695d022b0e442b/third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/accelerated-overflow-scroll-should-not-affect-perspective-expected.txt
[modify] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/Source/core/layout/LayoutBlockTest.cpp
[modify] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/Source/core/layout/LayoutBox.cpp
[modify] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/Source/core/layout/LayoutBox.h
[modify] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/Source/core/layout/LayoutTableRow.cpp
[modify] https://crrev.com/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3/third_party/WebKit/Source/core/layout/api/LineLayoutBox.h

Thanks, glad trchen was in our office when we were disussing overflow.

This will make the NG transition smoother.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/87eb92e903a8b3a0fb93617f813408eb5e53fedb

commit 87eb92e903a8b3a0fb93617f813408eb5e53fedb
Author: pdr <pdr@chromium.org>
Date: Mon Jul 10 21:37:58 2017

Mark overflow/overflow-transform-002.html as passing

This is passing after:
https://chromium.googlesource.com/chromium/src.git/+/602f4cdf9e0944a68fe91d12ead5b2ece186b8e3

Bug:  723788 
Change-Id: I1a322e8bd4641c33566794e287ae28d2c96a9f71
Reviewed-on: https://chromium-review.googlesource.com/565525
Commit-Queue: Philip Rogers <pdr@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485404}
[modify] https://crrev.com/87eb92e903a8b3a0fb93617f813408eb5e53fedb/third_party/WebKit/LayoutTests/TestExpectations