This bug is showing up in Chrome 63 as well. I can repro it on Mac/Chrome 63.0.3239.132; however, I cannot repro this on Mac/Chrome 63.0.3239.84 (I don't have information for any releases in between these two).

 Issue 802824  has been merged into this issue.

Sounds like there are some internal details in http://b/71605481

Directing to input triage.

I suspect this might be related to a finch trial. Can you provide the "variations" listed in chrome://version for the cases where it reproduces?  Does enabling chrome://flags/#enable-site-per-process effect the reproducibility at all?

Also are you able to see what the focused element is set to (document.activeElement) when it looses focus?  That might give some indication of who is stealing focus.

Repro on Mac/Chrome 63.0.3239.132, with variations:

c134752e-b8b72c88
3095aa95-3f4a17df
6c43306f-ca7d8d80
47e5d3db-3d47f4f4
1210a805-ecd831c
b1edbc38-c306c9a2
ba3f87da-45bda656
776de70c-eadfd437
79616653-3f4a17df
68812885-4d2fac87
684d1cdf-160692dc
f347910c-3d47f4f4
4b61504a-d25ea691
9773d3bd-f23d1dea
8e3b2dc5-93702590
9e5c75f1-1039a221
f79cb77b-3f4a17df
4ea303a6-196db7ac
d92562a9-ca7d8d80
90bcbadc-3f4a17df
25fc488a-4d2fac87
1c2f7bbf-3f4a17df
58a025e3-c2b41702
1bced4a3-90fa85cd
b2f0086-93053e47
ef25c1eb-3f4a17df
494d8760-6843eff2
f47ae82a-746c2ad4
3ac60855-3ec2a267
f296190c-24dd8f80
4442aae2-4ad60575
ed1d377-e1cc0f14
75f0f0a0-a5822863
e2b18481-bd104136
e7e71889-4ad60575
94e68624-803f8fc4
f141d4bc-65bced95
e9ce63c1-36ab09a2
da4aaa01-ca7d8d80


The bug continues to reproduce when enabling enable-site-per-process

I can also repro this on Chrome Canary (65.0.3323.0) with variations:

bd23585d-3f4a17df
c134752e-e80f7cd5
fe69e053-94941f92
16e0dd70-3f4a17df
f113d3c9-9b615b89
9041608a-3f4a17df
1e528f0f-ca7d8d80
b130ecb8-2e32ee7e
6025934e-3f4a17df
7c1bc906-b5809d46
d52c4ff7-d52c4ff7
3eb101d6-f23d1dea
47e5d3db-3d47f4f4
34d450b1-1410f10
19c1fdaf-3f4a17df
3042ad4b-ca7d8d80
57f575bb-3f4a17df
d0ecf1da-12de74c0
b72f69e9-3f4a17df
f347910c-65bced95
4b61504a-72dae5b3
77bbdddc-e08c81ee
5485fc4d-ca7d8d80
93731dca-e89d496c
8e3b2dc5-93702590
a428bf14-b23b1b55
9e5c75f1-e406a769
350fabdd-1cf80f06
9f6e28b-ea5ba878
2981bcb4-3d47f4f4
3de1fbf2-3f4a17df
f79cb77b-3f4a17df
47edb3-59ba3fef
4ea303a6-759f970c
bcc34a89-ca7d8d80
6e6e0c7e-f23d1dea
d92562a9-e1dd0eba
2b33233e-d8253d6f
14c5a050-ca7d8d80
6973a1cf-3f4a17df
72606c4f-f23d1dea
ad6d27cc-7075cd8
757a5d98-28165b59
23496387-4ea78229
b2f0086-11f17831
2d871858-f23d1dea
344833e9-1525b35b
4bc337ce-b2a7d6c5
57789a80-ca7d8d80
9a2f4e5b-ca7d8d80
3ac60855-3ec2a267
f296190c-eaceac83
4442aae2-6bdfffe7
ed1d377-e1cc0f14
75f0f0a0-d7f6b13c
e2b18481-e1cc0f14
e7e71889-4ad60575
34baa302-c306c9a2
f5fff3a2-3f4a17df
bbb8f811-3f4a17df
81c6897f-2d9ebb2e
9cade933-6a0b1f3c
493ac2c5-2a2a9269

Bisected it to these set of changes...
https://chromium.googlesource.com/chromium/src/+log/6a0943d812263288972d4197252f78386edb76de..9cf0ac139cc65890b9e036f704040c8da332d6e3

I've verified that adding this command line flag: "--disable-features=sign-in-process-isolation" brings back the old behavior.

Over to the site-isolation team, it seems relative to which frame has focus.