The problem: if user scrolled to the edge of the window, the state that
edge is reached, transfers to other windows, rendering user unable to
scroll further up/down in other windows as well.
Make unique states for each window that show that edge of the DB is reached.
Later it might be improved with MAM states as well to decrease load on program
in cases when user keeps scrolling despite reaching the edge.
Bug discovered by @atomicwatch, reported through the developers' MUC
`SCROLL_INNER` naming suggestion by @jubalh
0d29e05d77