dstockwell@'s previous comment: 
"If this is related to emoji then very likely font related -- probably an issue similar to https://bugs.chromium.org/p/chromium/issues/detail?id=470458"

mikelawther@ believes this should be looked at by font people. But before I dig deeper, rune@, would you have any idea about this one, since dstockwell@ points out you've worked on a similar issue, changing font-cache update triggers.

drott@, yes, the general problem is around fonts and operator== for ComputedStyle.

baseLayoutStyle is an optimization we have where we during an animation find out that the only style changes happening for an element is for an ongoing animation. We then store the ComputedStyle for the element without animations applied, and for every animation frame clone that computed style applying animations on top. Thus, we avoid doing selector matching and style application for the rest of the computed style.

With asserts enabled, we store the baseLayoutStyle, but do the full styleForElement to check that the optimization is sound. I've been through various fixes for ComputedStyle::operator==() where it would return false when the computed style conceptually is the same while there were some flags/members which were different, yet the optimization was sound.

Some of the font comparisons have been moved out of operator== and compared explicitly outside of that operator in places like comparing loading status for custom fonts [1].

I can not reproduce in Linux content_shell. Have you been able to reproduce with an android build?

I have just started to fetch for doing an android build.

[1] https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/Source/core/dom/Element.cpp&q=Element::recalcOw&sq=package:chromium&type=cs&l=1633

Thanks for looking into this and your explanations. 

So far I was only able to reproduce this on Android. Because of  issue 555855  I can only reproduce color emoji issues on Linux when I do a release build. On Android the assertions are enabled, on Linux they aren't. 

Looking into gn args --list, I will now try using dcheck_always_on = true on the Linux release build and try to reproduce there.

Unfortunately, I can't reproduce this on Linux even with "dcheck_always_on = true" and having verified that the #if ENABLE(ASSERT) parts of ElementAnimations.cpp are compiled in.

I've made a debug content shell apk which triggers this. Looking now.

Text auto sizing is changing the computed style under our feet.

Up for review: https://codereview.chromium.org/1816103002

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d

commit 32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d
Author: rune <rune@opera.com>
Date: Mon Mar 21 19:17:22 2016

Clear baseComputedStyle when text-autosizing changes.

baseComputedStyle is an optimization for animations where the computed
style before animations are applied is cached and cloned to have
cheaper style recalcs for per-frame animation changes. An assumption is
that the computed style for the layout object only changes in
styleForElement or pseudoStyleForElement. That assumption is not true
for text autosizing as the computed style may be changed during layout.
Then, for the next animation frame, the text autosizing factor may be
different even though the style has not been marked for recalc, and the
sanity check for an unchanged baseComputedStyle will trigger an assert.

Make sure we clear the baseComputedStyle for an element when the text
autosizing factor changes.

R=pdr@chromium.org,drott@chromium.org
BUG= 596018 

Review URL: https://codereview.chromium.org/1816103002

Cr-Commit-Position: refs/heads/master@{#382350}

[add] https://crrev.com/32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d/third_party/WebKit/LayoutTests/fast/text-autosizing/basecomputedstyle-assert-expected.txt
[add] https://crrev.com/32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d/third_party/WebKit/LayoutTests/fast/text-autosizing/basecomputedstyle-assert.html
[modify] https://crrev.com/32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d/third_party/WebKit/Source/core/layout/LayoutObject.cpp
[modify] https://crrev.com/32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d/third_party/WebKit/Source/core/layout/LayoutObject.h
[modify] https://crrev.com/32a7b19e92a9ff0fca3aba45c1fdd3896fdd5f1d/third_party/WebKit/Source/core/layout/TextAutosizer.cpp

Thanks!