Probably Safari on iPad. It seems to be specific for some comments. Maybe because some comment in that solution were deleted. The same issue occurs also with Edge browser. If a edit comment, in that solution, a bunch of editors are opened. Same thing with Internet Explorer 11. Same thing with Google Chrome 48. The problem really seems to be specific to some cases. In fact, it seems that the problem only occurs if I try to edit either a deleted comment or a comment after the deleted comment whatever the level. If we don't pay attention because other editor are below in the page, we can easily overwrite the wrong comment even from someone else... I just tried it again with 4 different browser and get the problem in each one. Try to edit any comment in Solution 3 from the first deleted comment and you should see the editor opened once for every comment starting at the deleted one until the last comment in that answer. I tried to find other questions with deleted comments (except last one on an answer) but it does not happen very often so at this point I was only able to reproduce the problem in that answer.
Philippe Mori