Arrange for keystrokes typed at FRAME to be sent to FOCUS-FRAME.
In other words, switch-frame events caused by events in FRAME will
request a switch to FOCUS-FRAME, and `last-event-frame' will be
FOCUS-FRAME after reading an event typed at FRAME.
If FOCUS-FRAME is nil, any existing redirection is canceled, and the
frame again receives its own keystrokes.
Focus redirection is useful for temporarily redirecting keystrokes to
a surrogate minibuffer frame when a frame doesn't have its own
A frame's focus redirection can be changed by `select-frame'. If frame
FOO is selected, and then a different frame BAR is selected, any
frames redirecting their focus to FOO are shifted to redirect their
focus to BAR. 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 redirected to nil; the former
is affected by `select-frame', while the latter is not.
The redirection lasts until `redirect-frame-focus' is called to change it. (fn FRAME &optional FOCUS-FRAME)