pre-redisplay-function

pre-redisplay-function is a variable defined in `C source code'.
Its value is

#[128 "\300\301\"\210\300\302\"\207"
      [apply redisplay--update-region-highlights ignore nil]
      4 nil nil]

• This variable may be risky if used as a file-local variable.

Documentation:
Function run just before redisplay.
It is called with one argument, which is the set of windows that are to
be redisplayed. This set can be nil (meaning, only the selected window),
or t (meaning all windows).