Windows7 x64
Open a file that has more than a screenful of lines and scroll down past the first screenful. Enter the 'move-lines_down' in the command line (I have it mapped to Alt-Down). The line gets moved correctly, but all of the text in the window gets scrolled down a line.
move-lines-up is different. It leaves the rest of the window as-is unless it needs to scroll to keep the current line visible.
In SE19, both move-lines-up and move-lines_down did the unnecessary scroll; SE20 seems to have corrected move-lines-up but not move-lines_down.
