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+;;; exwm-mff.el --- Mouse Follows Focus           -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2019, 2020, 2021  Ian Eure
+
+;; Author: Ian Eure <public@lowbar.fyi>
+;; URL: https://github.com/ieure/exwm-mff
+;; Version: 1.2.1
+;; Package-Requires: ((emacs "25.1"))
+;; Keywords: unix
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Mouse Follows Focus
+;; ===================
+;;
+;; Traditional window managers are mouse-centric: the window to receive
+;; input is usually selected with the pointing device.
+;;
+;; Emacs is keybord-centric: the window to receive key input is usually
+;; selected with the keyboard.  When you use the keyboard to focus a
+;; window, the spatial relationship between pointer and active window is
+;; broken -- the pointer can be anywhere on the screen, instead of over
+;; the active window, which can make it hard to find.
+;;
+;; The same problem also exists in traditional windowing systems when
+;; you use the keyboard to switch windows, e.g. with Alt-Tab.
+;;
+;; Because Emacs’ model is inverted, this suggests that the correct
+;; behavior is also the inverse -- instead of using the mouse to
+;; select a window to receive keyboard input, the keyboard should be
+;; used to select the window to receive mouse input.
+;;
+;; `EXWM-MFF-MODE' is a global minor mode which does exactly this.
+;; When the selected window in Emacs changes, the mouse pointer is
+;; moved to its center, unless the pointer is already somewhere inside
+;; the window’s bounds.  While it's especially helpful for for EXWM
+;; users, it works for any Emacs window in a graphical session.
+;;
+;; This package also offers the `EXWM-MFF-WARP-TO-SELECTED' command,
+;; which allows you to summon the pointer with a hotkey.  Unlike the
+;; minor mode, summoning is unconditional, and will place the pointer in
+;; the center of the window even if it already resides within its bounds
+;; -- a handy feature if you’ve lost your pointer, even if you’re using
+;; the minor mode.
+;;
+;;
+;; Limitations
+;; ~~~~~~~~~~~
+;;
+;; None known at this time.
+
+;;; Code:
+
+(require 'subr-x)
+(require 'cl-macs)
+
+(defcustom exwm-mff-ignore-if nil
+  "List of predicate functions for windows to ignore.
+
+Predicates accept one argument, WINDOW, and return non-NIL if
+automatic pointer warping should be suppressed."
+  :type 'hook
+  :group 'exwm-mff)
+
+(defconst exwm-mff--debug-buffer " *exwm-mff-debug*"
+  "Name of the buffer exwm-mff will write debug messages into.")
+
+(defvar exwm-mff--debug 0
+  "Whether (and how) to debug exwm-mff.
+0 = don't debug.
+1 = log messages to *exwm-mff-debug*.
+2 = log messages to *exwm-mff-debug* and the echo area.")
+
+(defvar exwm-mff--last-window nil
+  "The last selected window.")
+
+(defun exwm-mff--guard ()
+  "Raise an error unless this is a graphic session with mouse support."
+  (unless (and (display-graphic-p) (display-mouse-p))
+    (error "EXWM-MFF-MODE doesn't work on non-graphic or non-mouse sessions")))
+
+(defun exwm-mff--contains-pointer? (window)
+  "Return non-NIL when the mouse pointer is within FRAME and WINDOW."
+  (cl-destructuring-bind ((mouse-x . mouse-y) (left top right bottom))
+      (list (mouse-absolute-pixel-position)
+            (window-absolute-pixel-edges window))
+    (and (<= left mouse-x right)
+         (<= top mouse-y bottom))))
+
+(defun exwm-mff--debug (string &rest objects)
+  "Log debug message STRING, using OBJECTS to format it."
+  (let ((debug-level (or exwm-mff--debug 0)))
+    (when (> debug-level 0)
+      (let ((str (apply #'format (concat "[%s] " string)
+                        (cons (current-time-string) objects))))
+        (when (>= debug-level 1)
+          (with-current-buffer (get-buffer-create exwm-mff--debug-buffer)
+            (goto-char (point-max))
+            (insert (concat str "\n")))
+          (when (>= debug-level 2)
+            (message str)))))))
+
+(defun exwm-mff-show-debug ()
+  "Enable exwm-mff debugging, and show the buffer with debug logs."
+  (interactive)
+  (setq exwm-mff--debug 1)
+  (pop-to-buffer (get-buffer-create exwm-mff--debug-buffer)))
+
+(defun exwm-mff--window-center (window)
+  "Return a list of (x y) coordinates of the center of WINDOW in FRAME."
+  (cl-destructuring-bind (left top right bottom) (window-pixel-edges window)
+    (list (+ left (/ (- right left) 2))
+          (+ top (/ (- bottom top) 2)))))
+
+(defun exwm-mff-warp-to (frame window)
+  "Place the pointer in the center of WINDOW in FRAME."
+  (apply #'set-mouse-pixel-position frame
+         (exwm-mff--window-center window)))
+
+;;;###autoload
+(defun exwm-mff-warp-to-selected ()
+  "Place the pointer in the center of the selected window."
+  (interactive)
+  (exwm-mff--guard)
+  (exwm-mff-warp-to (selected-frame) (selected-window)))
+
+(defun exwm-mff--explain (selected-window same-window? contains-pointer? mini? ignored?)
+  "Use SELECTED-WINDOW, SAME-WINDOW?, CONTAINS-POINTER?, MINI?
+and IGNORED? to return an explanation of focusing behavior."
+  (cond
+   (same-window? "selected window hasn't changed")
+   (contains-pointer? "already contains pointer")
+   (mini? "is minibuffer")
+   (ignored? "one or more functions in `exwm-mff-ignore-if' matches")
+   (t (format "doesn't contain pointer (in %s)" selected-window))))
+
+(defun exwm-mff-hook (sw &optional norecord)
+  "EXWM-MFF-MODE hook.
+
+This is after-advice placed on SELECT-WINDOW.  It moves the
+pointer to SW (the currently selected window), if NORECORD is
+nil, and if it's not already in it."
+  (unless norecord
+    (if-let ((same-window? (eq sw exwm-mff--last-window)))
+        ;; The selected window is unchanged, we don't need to check
+        ;; anything else.
+        (exwm-mff--debug
+         "nop-> %s" (exwm-mff--explain sw same-window? nil nil nil))
+
+      (let* ((sf (window-frame sw))
+             (contains-pointer? (exwm-mff--contains-pointer? sw))
+             (mini? (minibufferp (window-buffer sw)))
+             (ignore? (run-hook-with-args-until-success 'exwm-mff-ignore-if sw)))
+        (if (or same-window? contains-pointer? mini? ignore?)
+            (exwm-mff--debug
+             "nop-> %s::%s (%s)" sf sw (exwm-mff--explain sw nil contains-pointer? mini? ignore?))
+          (exwm-mff--debug
+           "warp-> %s::%s (%s)" sf sw (exwm-mff--explain sw nil contains-pointer? mini? ignore?))
+          (exwm-mff-warp-to sf (setq exwm-mff--last-window sw)))))))
+
+(defgroup exwm-mff nil
+  "Mouse-Follows-Focus mode for EXWM."
+  :group 'exwm)
+
+;;;###autoload
+(define-minor-mode exwm-mff-mode
+  "Mouse follows focus mode for EXWM."
+  :global t
+  :require 'exwm-mff
+  :group 'exwm-mff
+  (exwm-mff--guard)
+  (if exwm-mff-mode
+      (advice-add 'select-window :after #'exwm-mff-hook)
+    (advice-remove 'select-window #'exwm-mff-hook)))
+
+(provide 'exwm-mff)
+
+;;; exwm-mff.el ends here