Menu Mouse Clicks Frames Creating Frames
You can use mouse clicks on window mode lines to select and manipulate windows.
Mouse-1 on a mode line selects the window above. By dragging Mouse-1 on the mode line, you can move it, thus changing the height of the windows above and below.
Mouse-2 on a mode line expands that window to fill its frame.
Mouse-3 on a mode line deletes the window above.
C-Mouse-2 on a mode line splits the window above horizontally, above the place in the mode line where you click.
C-Mouse-2 on a scroll bar splits the corresponding window vertically. See Split Window.Menu Mouse Clicks Frames Creating Frames