Thanks, rune, looking into it.

Firefox issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1351339

I'm experimenting with some required cleanup here: https://codereview.chromium.org/2778153003. Seems to work fine locally, waiting for a dry run now.

With that, the test-case doesn't work still. This is at least because of two different issues.

 * Hit testing or hover code seems to skip text nodes for this, so in the test case the hover node that arrives to the document is the body.
 * With an intermediate div, and verifying that the display: contents node is added to the path, the style doesn't yet change. I need to track that down further, but I need to run for today.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3f2db89d6d660abe00760144e734ccb9963cce2a

commit 3f2db89d6d660abe00760144e734ccb9963cce2a
Author: ecobos <ecobos@igalia.com>
Date: Mon Apr 03 13:30:00 2017

Use the flat tree for hover handling.

Modulo spec debate like[1], this is much simpler and closer to the spec than
what it was implemented.

This doesn't actually fix  issue 705984 , but it's a required cleanup, and I've
verified that the display: contents node is added to the :hover chain (if
there's an intermediate <div>), so I probably need to investigate how hit testing
and style invalidation is done, etc, to track down that bug.

Still this is worth landing IMO.

BUG= 705984 

Review-Url: https://codereview.chromium.org/2778153003
Cr-Commit-Position: refs/heads/master@{#461417}

[modify] https://crrev.com/3f2db89d6d660abe00760144e734ccb9963cce2a/third_party/WebKit/Source/core/dom/Document.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ad28574a6bf7b3ddce555b536c6047373b5727ab

commit ad28574a6bf7b3ddce555b536c6047373b5727ab
Author: rune <rune@opera.com>
Date: Wed Apr 05 20:22:35 2017

Allow display:contents elements in hover chain.

Document::updateHoverActiveState walked the shadow-including ancestor
path assuming no layoutObject meant display:none. Changed to walk flat
tree ancestors checking for display:contents style in addition.

ContainerNode::setHovered did not allow hover state to be changed when
setting hovered=true on elements without a layout object. Changed to
allow for display:contents here as well.

R=ecobos@igalia.com
BUG= 705984 

Review-Url: https://codereview.chromium.org/2790133002
Cr-Commit-Position: refs/heads/master@{#462199}

[add] https://crrev.com/ad28574a6bf7b3ddce555b536c6047373b5727ab/third_party/WebKit/LayoutTests/fast/css/hover-display-contents.html
[modify] https://crrev.com/ad28574a6bf7b3ddce555b536c6047373b5727ab/third_party/WebKit/Source/core/dom/ContainerNode.cpp
[modify] https://crrev.com/ad28574a6bf7b3ddce555b536c6047373b5727ab/third_party/WebKit/Source/core/dom/Document.cpp