Very interesting, but I have not faced with such problem this autumn while I was developing some contest project in RubyMine or probably I haven’t payed enough attention to it.

By default, RubyMine’s right arrow has the following behavior - move a caret to one symbol right.

And the problem here with end of lines - the carret doesn’t move to next line, it stays on same line and only moves one symbol right. Probably it could annoy you, and in this cases you can use Alt + -> or something else. Or you can change a behavior.

To change this behavior for carret just uncheck Allow placement of caret after end of line option in Editor section of Preferences.

PS. There are plenty of options available here in Preferences, so sometimes it could be hard to find first time exactly one you need.