Thanks for filing the issue!

Able to reproduce the issue on reported chrome version 71.0.3554.0 and on the latest canary 71.0.3559.0 using Windows 10, Ubuntu 14.04 and Mac 10.13.1

Flag, #enable-layout-ng is introduced in version 69.0.3482.0 and the issue is seen in this version too. Hence considering it as Non-Regression and marking it as Untriaged. Removing Needs-Bisect label and requesting someone from respective team to have a look into this and help in further triaging it.

This basically happens with any cr-dialog within Settings or Extensions, not just clear browsing data dialog.

Suspecting that this is related to the underlying <dialog> element positioning.

Yes, we fail to position the DIALOG element. It has non-auto top/bottom (50%/50%), so we refrain from applying any DIALOG magic to it. We fall back to generic out-of-flow positioning. Here we consider the element to be over-constrained, so we disregard bottom:50%. But we're not really over-constrained, because margins are auto, so the right thing to do, would have been to resolve those, rather than disregarding the bottom property.

Deleted: tc.html 276 bytes

tc.html 276 bytes View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/67ef0616d0d14b2e8931459d0a93cbcd63b03843

commit 67ef0616d0d14b2e8931459d0a93cbcd63b03843
Author: Morten Stenshorne <mstensho@chromium.org>
Date: Tue Oct 09 19:58:03 2018

[LayoutNG] Auto top/bottom margins may resolve to negative values.

The spec doesn't say that auto margin-top or margin-bottom need to
resolve to something non-negative, so just allow negative values as
well. They don't cause over-constrainedness.

Also, when distributing the space to margin-top and margin-bottom, make
sure that everything gets distributed. Set margin-top to the half, and
set margin-bottom to total space MINUS margin-top. Just dividing both by
two might cause us to lose a tiny fraction of the total, due to rounding.

Bug:  888270 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: Ia10577851e60707a0e2d67857a1c43a12e28b9d2
Reviewed-on: https://chromium-review.googlesource.com/c/1269855
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598052}
[add] https://crrev.com/67ef0616d0d14b2e8931459d0a93cbcd63b03843/third_party/WebKit/LayoutTests/external/wpt/css/CSS2/positioning/absolute-non-replaced-height-013.html
[modify] https://crrev.com/67ef0616d0d14b2e8931459d0a93cbcd63b03843/third_party/blink/renderer/core/layout/ng/ng_absolute_utils.cc
[modify] https://crrev.com/67ef0616d0d14b2e8931459d0a93cbcd63b03843/third_party/blink/renderer/core/layout/ng/ng_absolute_utils_test.cc