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.

etc/NEWS

@@ -575,6 +575,11 @@ 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

lisp/frame.el

@@ -129,7 +129,7 @@ appended when the minibuffer frame is created."
       ;; Gildea@x.org says it is ok to ask questions before terminating.
       (save-buffers-kill-emacs))))
 
-(defun handle-focus-in (_event)
+(defun handle-focus-in (&optional _event)
   "Handle a focus-in event.
 Focus-in events are usually bound to this function.
 Focus-in events occur when a frame has focus, but a switch-frame event
@@ -138,7 +138,7 @@ This function runs the hook `focus-in-hook'."
   (interactive "e")
   (run-hooks 'focus-in-hook))
 
-(defun handle-focus-out (_event)
+(defun handle-focus-out (&optional _event)
   "Handle a focus-out event.
 Focus-out events are usually bound to this function.
 Focus-out events occur when no frame has focus.
@@ -2339,7 +2339,8 @@ frame receives focus."
 This is done when a frame gets focus.  Blink timers may be stopped by
 `blink-cursor-suspend'."
   (when (and blink-cursor-mode
-	     (not blink-cursor-idle-timer))
+	     (not blink-cursor-idle-timer)
+             (display-graphic-p))
     (remove-hook 'post-command-hook 'blink-cursor-check)
     (blink-cursor--start-idle-timer)))
 

lisp/term/xterm.el

@@ -104,6 +104,16 @@ Return the pasted text as a string."
 
 (define-key global-map [xterm-paste] #'xterm-paste)
 
+(defun xterm-handle-focus-in ()
+  (interactive)
+  (handle-focus-in))
+(define-key global-map [xterm-focus-in] #'xterm-handle-focus-in)
+
+(defun xterm-handle-focus-out ()
+  (interactive)
+  (handle-focus-out))
+(define-key global-map [xterm-focus-out] #'xterm-handle-focus-out)
+
 (defvar xterm-rxvt-function-map
   (let ((map (make-sparse-keymap)))
     (define-key map "\e[2~" [insert])
@@ -136,6 +146,9 @@ Return the pasted text as a string."
     ;; internally recognizes the end.
     (define-key map "\e[200~" [xterm-paste])
 
+    (define-key map "\e[I" [xterm-focus-in])
+    (define-key map "\e[O" [xterm-focus-out])
+
     map)
   "Keymap of escape sequences, shared between xterm and rxvt support.")
 
@@ -817,6 +830,8 @@ We run the first FUNCTION whose STRING matches the input events."
   ;; Unconditionally enable bracketed paste mode: terminals that don't
   ;; support it just ignore the sequence.
   (xterm--init-bracketed-paste-mode)
+  ;; We likewise unconditionally enable support for focus tracking.
+  (xterm--init-focus-tracking)
 
   (run-hooks 'terminal-init-xterm-hook))
 
@@ -832,6 +847,12 @@ We run the first FUNCTION whose STRING matches the input events."
   (push "\e[?2004l" (terminal-parameter nil 'tty-mode-reset-strings))
   (push "\e[?2004h" (terminal-parameter nil 'tty-mode-set-strings)))
 
+(defun xterm--init-focus-tracking ()
+  "Terminal initialization for focus tracking mode."
+  (send-string-to-terminal "\e[?1004h")
+  (push "\e[?1004l" (terminal-parameter nil 'tty-mode-reset-strings))
+  (push "\e[?1004h" (terminal-parameter nil 'tty-mode-set-strings)))
+
 (defun xterm--init-activate-get-selection ()
   "Terminal initialization for `gui-get-selection'."
   (set-terminal-parameter nil 'xterm--get-selection t))