Multiple editor windows in Xcode

When coding in Xcode, I double-click a file in the "Groups and files" list, and it opens a new editor window. I have several of these open at once.

Until today.

Today, when I double-click a file in the list, it opens the file in the last editor window I was using, meaning I can 开发者_运维百科only have one editor window up at any one time.

I guess I've switched some option somewhere by mistake. How do I get it back to the way I like it?

I'm using Xcode 3.2.5

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In the top right of the editing window is an icon that says "grouped", perhaps you clicked on it by accident.

Click on it so it says "ungrouped", and you'll be back to opening multiple windows.

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perhaps preferences>General>Open Counterpart in same Editor