Support terminal focus notifications

* lisp/frame.el (handle-focus-in,handle-focus-out): Make event
argument optional.
(blink-cursor-check): Make sure that the current frame is a
window-system frame before restarting the blink timer. TTY frames
can get focus, but don't need a blink timer because the terminal
will do the blinking.

* lisp/term/xterm.el
(xterm-handle-focus-in,xterm-handle-focus-out): New functions.
(xterm-rxvt-function-map): Recognize focus notification sequences.
(xterm--init-focus-tracking): New function.
(terminal-init-xterm): Call it.

1 files changed, 5 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 46a57b1888c..3371718473f 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -576,6 +576,11 @@ manual for more details.
 * Lisp Changes in Emacs 27.1
 
 +++
+** Emacs now requests and recognizes focus-change notifications from
+   terminals that support the feature, meaning that `focus-in-hook'
+   and `focus-out-hook' may run for TTY frames.
+
++++
 ** Face specifications (of the kind used in `face-remapping-alist')
    now support filters, allowing faces to vary between windows display
    the same buffer.