The screen divider is the border between two non-overlapping windows. There are screen dividers between the object organizer and the of the main window, between the interface (declaration part) and the implementation (instruction part) of POUs and between the work space and the message window.
You can move the screen divider with the mouse pointer. You do this by moving the mouse with the left mouse button pressed.
Make sure the screen divider always remains at its absolute position, even when the window size has been changed. If it seems that the screen divider is no longer present, then simply enlarge your window.