I'm developing an objective-c app where I have several views which I want to show in the same window. I've done that programmatically before, but since Xcode 6 offers to use Storyboards for OS X apps, I want to implement that using Storyboard segues. But every segue type, except for the sheet segue, shows the new view only in the different window. Is there a good way to show the new view in the current window? Sheet segues do almost what I need, but they are not supposed to be used like that, as I know. You haven't specified whether you're invoking these segues programmatically or by a connection in Interface Builder. If you're doing so programmatically, it would be immensely helpful for you to post the code.
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If you're using Interface Builder, a screenshot of your storyboard would help. Without it, we can only guess. So, a guess: You're invoking a segue programmatically but are not doing so through NSViewController's documented methods for presentation of a child view controller whose ancestor is the content view controller of the window you intended to contain it. I haven't seen the behavior you're describing (a new window for each presented view controller, regardless of segue, unless it's a sheet segue, in which case it 'sort of' works) but I'm guessing whatever your method, you're not providing the key elements needed: a view controller with a window's-content-view-controller parent or a presenting view controller. Beyond that, it's hard to imagine anything else or anything more specific without more information.