Move a Window Splitter
To move a splitter between two panes in a window:
Hover over the window splitter.
The cursor changes into the horizontal 
or vertical 
splitter cursor. When the splitter cursor is active, click and hold the mouse button and drag:
to the left or right when dealing with a vertical splitter.
up or down when dealing with a horizontal splitter.
The panel size will be changed accordingly.
Note
Double-click the title bar of a viewer or use [Ctrl+M] or [Cmd+M] to toggle between maximizing or restoring down the active viewer.