This report applies to trunk r9671.

In the map editor, if you switch to unit mode and click the horizontal scroll bar above the palette that displays icons of units, the scroll bar gets the focus and indicates that by displaying a little white rectangle.  From then on, whenever you press the left or right arrow keys to scroll the map, the scroll bar also changes.  So far, it seems the only way to remove the focus from the scroll bar is to switch from unit mode to some other mode, and then optionally back to unit mode.