Before a window can receive input or other window management functions, you must select a window for input focus. To avoid ambiguity, only one window can be selected at a time. This window is said to be the active window. When a window is active, the color or shade of the window frame changes. You can choose the color or shading for active and inactive windows (see Chapter 5).
The input focus policy determines how a window is selected. The window manager supports both an explicit focus policy and an implicit focus policy. The implicit focus policy is also known as the pointer focus policy. The default input focus policy is explicit, which means that you must click Button 1 on the window frame or press Alt Tab to change the input focus from one window to another.
Pointer input focus policy causes mwm to change the input focus to match the pointer's position; no other actions are required. Thus, the window that contains the pointer always has the input focus. You can specify this input focus policy by modifying the defaults in the X configuration file. (See Chapter 5 for further information about setting input focus policy.)
The easiest way to select a window for input is to move the mouse so that the pointer rests within the frame of the window you want to use. If an explicit focus policy is in effect, you also need to click Button 1. With explicit focus policy, only the window selected for keyboard focus receives keyboard input, regardless of where the pointer is located. If you are using the explicit focus policy, you can also move the input focus using the keyboard. Pressing Alt Tab moves the input focus to the next window in the window stack; pressing Shift Alt Tab moves the input focus to the previous window.