CVar chatStyle

The style of Edit Boxes for the ChatFrame. Valid values: "classic", "im"

Default Setting

 * "im"

Possible Settings

 * "im":When clicking the tab of a Chat Frame, the entry box will attach to it. Each window's entry box will maintain its own saved state and history.
 * "classic":The chat entry box will work as it previously did. There will be a single entry box always attached to the primary Chat Frame.

Changing this setting
In-game, type the following /console chatStyle setting

Alternatively, you can exit wow, open the Config.wtf file, and add the following line: SET chatStyle setting