Steps to reproduce:
- visit https://en.wikipedia.beta.wmflabs.org/wiki/User:Tgr/sandbox?veaction=editsource (the bug seems to be specific to opening it via URL and not the edit button)
- click outside the edit box (defocus it)
- press ctrl-a - this will select everything in the page (not just in the editor, since the editing area is not focused at this point)
- click into the edit box
- press ctrl-a (this will select the wikitext only)
- press del
The text will be deleted but NWE won't notice the change (save button stays grey, switching to VE will show old text).
I can reproduce this most of the time but not reliably. I think it depends on not waiting too much between the steps. (Could be something that's totally unrelated to the while selecting business, but that's how I noticed it.)
Tested with Chrome 55 / Ubuntu 16.04.