I've Storyboard in editor on left pane and assitant editor on right pane. When I click on Jump to definition it doesn't open file in assitant editor, it opens in Editor (left pane where I've storyboard) instead. It's a random issue and happens most of time.
The way I open jump to definition is to right click on a function call and select Jump to definition. I have always used it so keyboard shortcut is not an issue.
It used to work better in earlier version of xCode. Is there any bug in xCode?