What's wrong exactly? Obviously with the cursor you can go to the both ends of the text.

I guess that the default state (without clicking anywhere) should be that you can see the beginning of the username, not the end, if it is very long.

Correct.

Screenshot for Linux (the same for Win/CromeOS)

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	username_clipping.png 36.6 KB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/76d6549b4e0c7fcc3d50def0a21f3fe4117510f2

commit 76d6549b4e0c7fcc3d50def0a21f3fe4117510f2
Author: Maxim Kolosovskiy <kolos@chromium.org>
Date: Thu Oct 19 15:30:52 2017

[Password Manager] Fix clipping of username value in a save prompt (Win)

In case of long username, the beginning of value should be visible.

Bug:  770055 
Change-Id: I09423d0131e7198de69211a020e38eaff2b7c503
Reviewed-on: https://chromium-review.googlesource.com/728099
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510091}
[modify] https://crrev.com/76d6549b4e0c7fcc3d50def0a21f3fe4117510f2/chrome/browser/ui/views/passwords/manage_password_items_view.cc

Verified the latest behavior on Windows-10, Mac OS 10.12.6 and Linux Ubuntu 14.04(64.0.3247.0). In case of long username, the beginning of value is visible now.

Screenshot attached. 

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	770055.png 82.6 KB View Download

Thanks for confirming.