But wait, what s an output flush rectangle?

It is a snippet uto itself, it has no functional relationship with IO manager, except IO manager dumps commands into it for scroll. But so does everyone else. Nor does the flush window have any active duties, when it is filled, someone calls the flush routine. So,, flush window is a simple snippet. It opens a fifo, available for writes. Then it has one entry, the function will read lines from the fifo and place them properly in the specified flush rectangle.  A much easier load for io manager which now only has to edit one 80 char single height window.  It is not bothered by rinf redirection at ll and dumps its stuff into write buffer like everyone else.