When dragging bottom-up, I’m often times seeing the left-most character erroneously selected. It seems this bug has been around for awhile now, e.g. https://discourse-cdn-sjc1.com/business6/uploads/github_atom/420/c4c7b503a9142197.gif
And my own document I made before running into the animated gif showing the bug back in 2014:
I drag-select all the time and am constantly having to fix curly braces that are missing. I will note that I recently switched to OSX and Atom from Windows and Sublime Text, so I’m not totally certain if this is a bug limited to three-finger dragging in OSX (which seems buggy) or a bug specific to Atom.