The bug depends on the cursor position (see image #2).
To reproduce the bug:
1) Open an empty document.
2) Type a short sentence.
3) Add a comment to one of the words.
4) Move the cursor to the "hidden space" which marks the end of the comment
Created attachment 81306 [details]
no bug, watch the position of the cursor!
Created attachment 81307 [details]
bug, the comment has the focus
I can't reproduce this bug in Libreoffice 18.104.22.168 rc on Ubuntu 13.04 x64 with GTK theme. Maybe this issue occurs only with Oxygen theme?
I tested it again, now using Raleigh as GTK2 Theme. Same result for me.
It's very important that the cursor is at the right position as described in #c1. Otherwise the bug's not triggered.
I reproduce this behaviour with master LO 22.214.171.124.alpha0+
Build ID: ce0383d2950d3b0da2c457c8a406c1229de53571 & Windows 7 Home Premium.
But I have to select the very thin space behind the word to get it.
In this case, Comments button is moved to the left side of the ruler, which is displayed without boundary on the right side.
Moving the cursor in another place give again the right behaviour for ruler.
So I set status for this report to NEW, Platform to All and minorize Importance to LOW TRIVIAL because it doesn't prevent to make high quality work.
Thank you for reporting.
Have a nice day,
Just another oberservation in 4.1.1: The bug only appears on comments with text range.
I think that the bug has been fixed automagically. At least I cannot reproduce it anymore with LO 126.96.36.199.