Xianzhu, could you look at this? Painting/compositing of collapsed borders is failing in this test, which you know way better than anyone else.

The top and left borders of the blue cell are not visible.

ping wangxianzhu@ ? any chance you have bandwidth for this?

Haven't got chance to work on this yet, but I can keep this in my task list. I can probably work on this in the next month.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9138bc279c1213164250d8532675b495d8fc0bba

commit 9138bc279c1213164250d8532675b495d8fc0bba
Author: Morten Stenshorne <mstensho@chromium.org>
Date: Thu Oct 18 23:00:19 2018

[LayoutNG] Self-painting border-collapsed table cells need to clip.

In the collapsed table borders model, all borders (from the table
itself, table sections, table columns, table rows and table cells) are
painted in the background-and-borders phase of the table. We need
special code to prevent the background of a self-painting-layer
table-cell from overlapping with the borders that have already been
painted. Similar code exists for legacy in TableCellPainter.

Bug:  869878 
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I190dd6a152e751f6b30409104fb466ca46bb9024
Reviewed-on: https://chromium-review.googlesource.com/c/1289890
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600959}
[modify] https://crrev.com/9138bc279c1213164250d8532675b495d8fc0bba/third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-features=LayoutNG
[modify] https://crrev.com/9138bc279c1213164250d8532675b495d8fc0bba/third_party/blink/renderer/core/paint/ng/ng_box_fragment_painter.cc