UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0

Steps to reproduce the problem:
1. Visit site with arabic keyboard implementation like ordbogen.com, created a simple mockup on https://jsfiddle.net/u62ewwab/7/
2. The input box will contain the last text searched (a value), in this case some arabic letters
3. Empty the input box, by deleting the current content
4. Now use the buttons to enter arabic letters
5. The letters are now added left to right and not right to left as expected. If you double click a letter very fast it seems to restore the state of selectionStart and selectionEnd and they will now update correctly.

What is the expected behavior?
The selectionStart and selectionEnd needs to correctly track their values between js call.

What went wrong?
When ever a button is clicked the console log shows the position of the selectionStart and selectionEnd of the input box. This position should normally increase as you enter more letters but in this case it alternates like so:
'click letter button'
  start: 0 0
  end 1 1
'click letter button'
  start: 0 0
  end 1 1
'click letter button'
  start: 0 0
  end 1 1
'click letter button'
  start: 0 0
  end 1 1

Instead of the correct case where the numbers increase
'click letter button'
  start: 0 0
  end 1 1
'click letter button'
  start: 1 1
  end 2 2
'click letter button'
  start: 2 2
  end 3 3
'click letter button'
  start: 3 3
  end 4 4

Did this work before? Yes Unknown - Im fairly sure it worked in version 55.* but was unable to find the version for regression testing

Chrome version: Version 61.0.3163.100 (Official Build) (64-bit)  Channel: stable
OS Version: Debian Jession 8.6
Flash Version: Shockwave Flash 10.1 r999.<br>Gnash 0.8.11dev, the GNU SWF Player.   Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011   <a href="http://www.fsf.org">Free   Software Foundation</a>, Inc. <br>   Gnash comes with NO WARRANTY, to the extent permitted by law.   You may redistribute copies of Gnash under the terms of the   <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public   License</a>. For more information about Gnash, see <a   href="http://www.gnu.org/software/gnash/">   http://www.gnu.org/software/gnash</a>.   <br>  Compatible Shockwave Flash 10.1 r999.

The issue was reported by users on ordbogen.com, and was found only to be a problem on newer versions of chrome.
It seems to be platform independent, since its the same problem on both Windows, Linux and Mac.