The elisp manual "Window Internals" has

    `redisplay_end_trigger'
         If redisplay in this window goes beyond this buffer position, it
         runs the `redisplay-end-trigger-hook'.

but is the variable in fact redisplay-end-trigger-functions?

2009-05-27  Kevin Ryde  <user42@zip.com.au>

	* internals.texi (Window Internals): Correction
	redisplay-end-trigger-functions, not redisplay-end-trigger-hook.