You are probably looking for a change made after 489362 (known good), but no later than 489363 (first known bad).

https://chromium.googlesource.com/chromium/src/+log/17684e9d8dd252a678cc17989303eaf027cbb61a..7a66627f0f0a681fcf3111a9bb0f01a8e0790186

When I add style="user-select:text" to <input readonly> in the sample.
I can select content.

Proposed fix is add inline style "user-selection:text !important" to 
inner editor of INPUT.

This remains broken in 63.3210

<textarea readonly> has the same bug.

The regression is caused by my change in html.css in https://chromium-review.googlesource.com/c/chromium/src/+/570246/16/third_party/WebKit/Source/core/css/html.css at line 426 and 498.

Here's a fix: https://chromium-review.googlesource.com/c/chromium/src/+/663217 .

 Issue 764051  has been merged into this issue.

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 Issue 766684  has been merged into this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ba7c210328c00a6693ddc5da6504f8520bb3bdc2

commit ba7c210328c00a6693ddc5da6504f8520bb3bdc2
Author: Hugo Holgersson <hugoh@opera.com>
Date: Fri Sep 22 17:59:09 2017

Ignore <input> and <textarea>'s user-select styling

Background:
The CSS spec says that user-select does "selective inheritance",
see https://drafts.csswg.org/css-ui-4/#propdef-user-select.

<input> and <textarea> are two elements that are affected
by this. These elements should not inherit user-select from
parent elements.

Fixed problem:
<input|textarea readonly|disabled> could inherit
"user-select: none".

This regressed because
https://chromium-review.googlesource.com/570246
removed input|textarea's default user-select styling.

Solution:
Make all text controls always selectable. This is done in
LayoutTextControl::AdjustInnerEditorStyle.

Interoperability note:
<input readonly style="user-select: none"
value="Chrome 61 cannot select this.">. This CL fixes
this by ensuring that all text fields (i.e also readonly fields)
are always selectable (no matter user-select styling).

In other words, <input> and <input readonly> now behave in
the same way (both always allow selections) which aligns
us with Firefox and Edge.

BUG= 761433 , 764316

Change-Id: I89fc94a2a04caf3a87b12a9071081dafd48a8727
Reviewed-on: https://chromium-review.googlesource.com/663217
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hugo Holgersson <hugoh@opera.com>
Cr-Commit-Position: refs/heads/master@{#503793}
[modify] https://crrev.com/ba7c210328c00a6693ddc5da6504f8520bb3bdc2/third_party/WebKit/Source/core/input/EventHandlerTest.cpp
[modify] https://crrev.com/ba7c210328c00a6693ddc5da6504f8520bb3bdc2/third_party/WebKit/Source/core/layout/LayoutTextControl.cpp

Issue 767433 has been merged into this issue.

This bug requires manual review: M62 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), bhthompson@(ChromeOS), abdulsyed@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Can you please confirm why this needs to be merged for M62? Can we wait until M63?
Has this been well tested in Canary?

I believe this needs to go into both M62 and M63 actually. 

Key web apps are blocked on it, for example https://forms.google.com/create and http://pdftron.com/webviewer/bugdemo/ .

Not sure if it has been tested in Canary yet. I could wait with backporting a few more days, if you prefer? But I believe it is better to backport sooner than later.

This bug requires manual review: We don't branch M63 until 2017-10-12.
Please contact the milestone owner if you have questions.
Owners: amineer@(Android), cmasso@(iOS), gkihumba@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Commit ba7c2103 landed in Canary in version 63.0.3223.0 on 9/24; no merge is needed.

I've verified the fix in 3223.

See #16 for merge justification for M62 (broken web apps).

Thanks for confirming. Approving merge to M62, branch:3202. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b01af0b296ecb855aac95c4ed335d188e6eac2de

commit b01af0b296ecb855aac95c4ed335d188e6eac2de
Author: Hugo Holgersson <hugoh@opera.com>
Date: Tue Sep 26 19:17:54 2017

Ignore <input> and <textarea>'s user-select styling

Background:
The CSS spec says that user-select does "selective inheritance",
see https://drafts.csswg.org/css-ui-4/#propdef-user-select.

<input> and <textarea> are two elements that are affected
by this. These elements should not inherit user-select from
parent elements.

Fixed problem:
<input|textarea readonly|disabled> could inherit
"user-select: none".

This regressed because
https://chromium-review.googlesource.com/570246
removed input|textarea's default user-select styling.

Solution:
Make all text controls always selectable. This is done in
LayoutTextControl::AdjustInnerEditorStyle.

Interoperability note:
<input readonly style="user-select: none"
value="Chrome 61 cannot select this.">. This CL fixes
this by ensuring that all text fields (i.e also readonly fields)
are always selectable (no matter user-select styling).

In other words, <input> and <input readonly> now behave in
the same way (both always allow selections) which aligns
us with Firefox and Edge.

BUG= 761433 , 764316
TBR=hugoh@opera.com

(cherry picked from commit ba7c210328c00a6693ddc5da6504f8520bb3bdc2)

Change-Id: I89fc94a2a04caf3a87b12a9071081dafd48a8727
Reviewed-on: https://chromium-review.googlesource.com/663217
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Hugo Holgersson <hugoh@opera.com>
Cr-Original-Commit-Position: refs/heads/master@{#503793}
Reviewed-on: https://chromium-review.googlesource.com/685655
Reviewed-by: Hugo Holgersson <hugoh@opera.com>
Cr-Commit-Position: refs/branch-heads/3202@{#453}
Cr-Branched-From: fa6a5d87adff761bc16afc5498c3f5944c1daa68-refs/heads/master@{#499098}
[modify] https://crrev.com/b01af0b296ecb855aac95c4ed335d188e6eac2de/third_party/WebKit/Source/core/input/EventHandlerTest.cpp
[modify] https://crrev.com/b01af0b296ecb855aac95c4ed335d188e6eac2de/third_party/WebKit/Source/core/layout/LayoutTextControl.cpp