Strange behavior of cursor when static scroll used

[slavazanko]

Test case:

• touch test.txt; for i in {0..300};do echo $i >>test.txt;done
• mcedit test.txt
• Press PageDown?; then press PageDown? again
• move cursor at middle of screen
• press Ctrl+Dowm

What is the expected:
Cursor must be holded at middle of screen, lines at all screen must be scrolled up

What see instead:
Cursor moved down, lines at all screen scrolled down.