5.6 How can I modify the titlebar to contain the current file name?

The contents of an Emacs frame's titlebar is controlled by the variable frame-title-format, which has the same structure as the variable mode-line-format. (Use C-h v or M-x describe-variable to get information about one or both of these variables.)

By default, the titlebar for a frame does contain the name of the buffer currently being visited, except if there is a single frame. In such a case, the titlebar contains Emacs invocation name and the name of the machine at which Emacs was invoked. This is done by setting frame-title-format to the default value of

     (multiple-frames "%b" ("" invocation-name "@" system-name))

To modify the behavior such that frame titlebars contain the buffer's name regardless of the number of existing frames, include the following in your .emacs:

     (setq frame-title-format "%b")