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At any time, one frame in Emacs is the selected frame. The selected window always resides on the selected frame.
When Emacs displays its frames on several terminals (see section 29.2 Multiple Displays), each terminal has its own selected frame. But only one of these is "the selected frame": it's the frame that belongs to the terminal from which the most recent input came. That is, when Emacs runs a command that came from a certain terminal, the selected frame is the one of that terminal. Since Emacs runs only a single command at any given time, it needs to consider only one selected frame at a time; this frame is what we call the selected frame in this manual. The display on which the selected frame is displayed is the selected frame's display.
Some window systems and window managers direct keyboard input to the window object that the mouse is in; others require explicit clicks or commands to shift the focus to various window objects. Either way, Emacs automatically keeps track of which frame has the focus.
Lisp programs can also switch frames "temporarily" by calling the
function select-frame. This does not alter the window system's
concept of focus; rather, it escapes from the window manager's control
until that control is somehow reasserted.
When using a text-only terminal, only the selected terminal frame is
actually displayed on the terminal. switch-frame is the only way
to switch frames, and the change lasts until overridden by a subsequent
call to switch-frame. Each terminal screen except for the
initial one has a number, and the number of the selected frame appears
in the mode line before the buffer name (see section 23.3.2 Variables Used in the Mode Line).
In general, you should never use select-frame in a way that could
switch to a different terminal without switching back when you're done.
Emacs cooperates with the window system by arranging to select frames as
the server and window manager request. It does so by generating a
special kind of input event, called a focus event, when
appropriate. The command loop handles a focus event by calling
handle-switch-frame. See section 21.6.9 Focus Events.
Focus events normally do their job by invoking this command. Don't call it for any other reason.
last-event-frame will be focus-frame. Also, switch-frame
events specifying frame will instead select focus-frame.
If focus-frame is nil, that cancels any existing
redirection for frame, which therefore once again receives its own
events.
One use of focus redirection is for frames that don't have minibuffers. These frames use minibuffers on other frames. Activating a minibuffer on another frame redirects focus to that frame. This puts the focus on the minibuffer's frame, where it belongs, even though the mouse remains in the frame that activated the minibuffer.
Selecting a frame can also change focus redirections. Selecting frame
bar, when foo had been selected, changes any redirections
pointing to foo so that they point to bar instead. This
allows focus redirection to work properly when the user switches from
one frame to another using select-window.
This means that a frame whose focus is redirected to itself is treated
differently from a frame whose focus is not redirected.
select-frame affects the former but not the latter.
The redirection lasts until redirect-frame-focus is called to
change it.
nil says that it does.
When this is so, the command other-frame moves the mouse to a
position consistent with the new selected frame.
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