The icon for switching between horizontal and vertical switching is constantly flashing. If you have a DMD2 T 865X tablet and you accidentally touch this icon, the navigation will change to vertical, but the icon for switching back to horizontal will disappear and the arrow will be on the right side of the display. So you have to stop and unlock the display. You have to manually rotate the display so that the icon for switching between horizontal and vertical positions appears again. The icon is located next to the zoom icon, so this can happen quite often.