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  In particular, they can go off when Scroll Lock etc have frozen the text
  output.
Detail:
  The callback chain is examined at least once per line scroll. Additionally,
  it is examined repeatedly while Scroll Lock or Ctrl-Shift are acting, while
  idling between lines if Ctrl is held down, and while waiting for the Shift
  key to be pressed at the bottom of a screen when in page mode.
  The opportunity is also taken to call Portable_Idle after each time that the
  callback chain has been examined.
Admin:
  Not tested.

Version 5.27. Tagged as 'Kernel-5_27'