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### 27.27 Hooks for Window Scrolling and Changes

This section describes how a Lisp program can take action whenever a window displays a different part of its buffer or a different buffer. There are three actions that can change this: scrolling the window, switching buffers in the window, and changing the size of the window. The first two actions run `window-scroll-functions`; the last runs `window-size-change-functions`.

Variable: window-scroll-functions

  This variable holds a list of functions that Emacs should call before redisplaying a window with scrolling. Displaying a different buffer in the window also runs these functions.

  This variable is not a normal hook, because each function is called with two arguments: the window, and its new display-start position.

  These functions must take care when using `window-end` (see Window Start and End); if you need an up-to-date value, you must use the update argument to ensure you get it.

  Warning: don’t use this feature to alter the way the window is scrolled. It’s not designed for that, and such use probably won’t work.

Variable: window-size-change-functions

  This variable holds a list of functions to be called if the size of any window changes for any reason. The functions are called at the beginning of a redisplay cycle, and just once for each frame on which size changes have occurred.

  Each function receives the frame as its sole argument. There is no direct way to find out which windows on that frame have changed size, or precisely how. However, if a size-change function records, at each call, the existing windows and their sizes, it can also compare the present sizes and the previous sizes.

  Creating or deleting windows counts as a size change, and therefore causes these functions to be called. Changing the frame size also counts, because it changes the sizes of the existing windows.

  You may use `save-selected-window` in these functions (see Selecting Windows). However, do not use `save-window-excursion` (see Window Configurations); exiting that macro counts as a size change, which would cause these functions to be called over and over.

Variable: window-configuration-change-hook

  A normal hook that is run every time you change the window configuration of an existing frame. This includes splitting or deleting windows, changing the sizes of windows, or displaying a different buffer in a window.

  The buffer-local part of this hook is run once for each window on the affected frame, with the relevant window selected and its buffer current. The global part is run once for the modified frame, with that frame selected.

In addition, you can use `jit-lock-register` to register a Font Lock fontification function, which will be called whenever parts of a buffer are (re)fontified because a window was scrolled or its size changed. See Other Font Lock Variables.

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