Last change to the GitHub master branch WiFi Addon was 22 days ago. I wonder if the cheapo ESP8266 flash memory misses writing a bit here and there sometimes.
I use the same Arduino IDE version.
Normally the OnStep.ino file is in the left most tab in the IDE, I don't see it there in your screen shot.
I see the directory name enclosing OnStep is called "Onstep" not "OnStep"... windows file system isn't case sensitive but perhaps that's tripping up the IDE.
