Able to reproduce the issue on Mac 10.11.6 using chrome version 55.0.2883.18 

Using the per-revision bisect providing the bisect results,
Good build:54.0.2795.0(Revision: 405182).
Bad build: 54.0.2796.0 (Revision:405391).

You are probably looking for a change made after 405246 (known good), but no later than 405248 (first known bad).

CHANGELOG URL:

The script might not always return single CL as suspect as some perf builds might get missing due to failure.
 
https://chromium.googlesource.com/chromium/src/+log/7206bff55954e76834cf8bcb8e9fab255abfacbe..6c8e8a9e0b13fd13fe29e310763a76d0cbc2221a

From the above CL list , suspecting the below change, 

@dpapad - Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.

Review-Url: https://codereview.chromium.org/2141063003

Thanks !

This seems to be a blink bug. Minimal repro at https://jsfiddle.net/7d9cx27t/4/.

@yosin: Do you know who could take a look at this? This is affecting Chrome's new material design page.

It seems 56.0.2899.3 (canary) works fine; I coped version number from chrome:/mid-settings/help with double-click version number.

yoichio@, according to a test case #5, it seems user-select causes this issue.

@yosin: Are you an a Mac? I am still able to reproduce on Linux on ToT, see attachment.

Deleted: user_select_bug.mp4 151 KB

	user_select_bug.mp4 151 KB View Download

dpapad@, I'm using Windows. I could reproduce this.
Double click at space between "Version" and "56.0...."
Here is simple test case: https://jsfiddle.net/q0avpz2k/

It seems this is "By Design" for double-clicking/tapping at space to select line.

I could reproduce this on 54.0.2840.71 (Official Build) m (64-bit)

To click double to select line is another topic, I think.
Repro step is:
(1)Launch chrome and go to chrome://md-settings/help >> 'Triple' click on version number field so that whole line gets selected
(2)Now copy text and try pasting it somewhere[Paste it in Omnibox] and observe.

Expected:
 Only 'Version 56.0.2905.0 canary (64-bit)' is pasted.

Actual
 'Version 56.0.2905.0 canary (64-bit) Get help with Chrome Report an issue' is pasted.
Appended text comes from following elements under version string.

Just to update the latest behavior of the bug, Issue is still observed on chrome version 55.0.2883.52.
yoichio@ Could you please let us know is there any recent update available on this issue? 

Thanks!

We can select only the version string by D&D selection.
The problem happens by triple click.
Selecting a line with triple click in Shadow DOM is not implemented yet.

This works as expected. 

Triple-click attempts to select paragraph and "version" to end of "Report and issue" are selected; note RELAUNCH, "Get help with Chrome", and "Report and Issue" have user-select:none.