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### 7.3 Window and Frame Configuration

On a non-windowing display, Ediff sets things up in one frame, splitting it between a small control window and the windows for buffers A, B, and C. The split between these windows can be horizontal or vertical, which can be changed interactively by typing `|` while the cursor is in the control window.

On a window display, Ediff sets up a dedicated frame for Ediff Control Panel and then it chooses windows as follows: If one of the buffers is invisible, it is displayed in the currently selected frame. If a buffer is visible, it is displayed in the frame where it is visible. If, according to the above criteria, the two buffers fall into the same frame, then so be it—the frame will be shared by the two. The same algorithm works when you type `C-l` (`ediff-recenter`), `p` (`ediff-previous-difference`), `n` (`ediff-next-difference`), etc.

The above behavior also depends on whether the current frame is splittable, dedicated, etc. Unfortunately, the margin of this book is too narrow to present the details of this remarkable algorithm.

The upshot of all this is that you can compare buffers in one frame or in different frames. The former is done by default, while the latter can be achieved by arranging buffers A, B (and C, if applicable) to be seen in different frames. Ediff respects these arrangements, automatically adapting itself to the multi-frame mode.

Ediff uses the following variables to set up its control panel (a.k.a. control buffer, a.k.a. quick help window):

`ediff-control-frame-parameters`
  You can change or augment this variable including the font, color, etc. The X resource name of Ediff Control Panel frames is ‘`Ediff`’. Under X-windows, you can use this name to set up preferences in your `~/.Xdefaults`, `~/.xrdb`, or whatever X resource file is in use. Usually this is preferable to changing `ediff-control-frame-parameters` directly. For instance, you can specify in `~/.Xdefaults` the color of the control frame using the resource ‘`Ediff*background`’.

  In general, any X resource pertaining the control frame can be reached via the prefix `Ediff*`.

`ediff-control-frame-position-function`
  The preferred way of specifying the position of the control frame is by setting the variable `ediff-control-frame-position-function` to an appropriate function. The default value of this variable is `ediff-make-frame-position`. This function places the control frame in the vicinity of the North-East corner of the frame displaying buffer A.

The following variables can be used to adjust the location produced by `ediff-make-frame-position` and for related customization.

`ediff-narrow-control-frame-leftward-shift`
  Specifies the number of characters for shifting the control frame from the rightmost edge of frame A when the control frame is displayed as a small window.
`ediff-wide-control-frame-rightward-shift`
  Specifies the rightward shift of the control frame from the left edge of frame A when the control frame shows the full menu of options.
`ediff-control-frame-upward-shift`
  Specifies the number of pixels for the upward shift of the control frame.
`ediff-prefer-iconified-control-frame`
  If this variable is `t`, the control frame becomes iconified automatically when you toggle the quick help message off. This saves valuable real estate on the screen. Toggling help back will deiconify the control frame.

  To start Ediff with an iconified Control Panel, you should set this variable to `t` and `ediff-prefer-long-help-message` to `nil` (see Quick Help Customization). This behavior is useful only if icons are allowed to accept keyboard input (which depends on the window manager and other factors).

To make more creative changes in the way Ediff sets up windows, you can rewrite the function `ediff-setup-windows`. However, we believe that detaching Ediff Control Panel from the rest and making it into a separate frame offers an important opportunity by allowing you to iconify that frame. The icon will usually accept all of the Ediff commands, but will free up valuable real estate on your screen (this may depend on your window manager, though).

The following variable controls how windows are set up:

`ediff-window-setup-function`
  The multiframe setup is done by the `ediff-setup-windows-multiframe` function, which is the default on windowing displays. The plain setup, one where all windows are always in one frame, is done by `ediff-setup-windows-plain`, which is the default on a non-windowing display (or in an xterm window). In fact, under Emacs, you can switch freely between these two setups by executing the command `ediff-toggle-multiframe` using the Minibuffer of the Menubar. If you don't like any of these setups, write your own function. See the documentation for `ediff-window-setup-function` for the basic guidelines. However, writing window setups is not easy, so you should first take a close look at `ediff-setup-windows-plain` and `ediff-setup-windows-multiframe`.

You can run multiple Ediff sessions at once, by invoking Ediff several times without exiting previous Ediff sessions. Different sessions may even operate on the same pair of files.

Each session has its own Ediff Control Panel and all the regarding a particular session is local to the associated control panel buffer. You can switch between sessions by suspending one session and then switching to another control panel. (Different control panel buffers are distinguished by a numerical suffix, e.g., ‘`Ediff Control Panel<3>`’.)