From 4b1ec3956117a0ed30f5df3bd19d71fb9ba51a48 Mon Sep 17 00:00:00 2001
From: Po Lu <luangruo@yahoo.com>
Date: Fri, 5 Aug 2022 13:21:29 +0800
Subject: [PATCH] Improve multi-pointer X support

* etc/TODO: Document that MPX support still needs work.
* src/xterm.c (xi_handle_interaction): New function.
(handle_one_xevent): Call it in the appropriate places.
---
 etc/TODO    | 13 +++++++++++++
 src/xterm.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 63 insertions(+)

diff --git a/etc/TODO b/etc/TODO
index 4d0bfd1c2a0..bd846dd8988 100644
--- a/etc/TODO
+++ b/etc/TODO
@@ -1746,6 +1746,19 @@ for vc-rcs-update-changelog.
 A two-char comment-starter whose two chars are symbol constituents will
 not be noticed if it appears within a word.
 
+** Multi-pointer X does not work very well
+Emacs is reasonably usable under a multi-pointer X (MPX) environment,
+if you turn off tooltips and mouse highlight, and don't use anything
+that calls `mouse-position'.  Otherwise, tooltips are shown next to
+the wrong pointer, mouse highlight simply goes haywire, and
+`mouse-position' returns information for the wrong pointer.
+
+This could be easily fixed in principle, but I cannot find a stable
+enough environment under which the fix can be tested.
+
+The MPX code has not been tested under X toolkit or GTK+ 2.x builds
+and is not expected to work there.
+
 
 This file is part of GNU Emacs.
 
diff --git a/src/xterm.c b/src/xterm.c
index ebd27aea069..4bbcfb0e596 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -12616,6 +12616,36 @@ xi_handle_delete_frame (struct x_display_info *dpyinfo,
     }
 }
 
+/* Handle an interaction by DEVICE on frame F.  TIME is the time of
+   the interaction; if F isn't currently the global focus frame, but
+   is the focus of DEVICE, make it the focus frame.  */
+
+static void
+xi_handle_interaction (struct x_display_info *dpyinfo,
+		       struct frame *f, struct xi_device_t *device,
+		       Time time)
+{
+  bool change;
+
+  change = false;
+
+  if (device->focus_frame == f)
+    {
+      device->focus_frame_time = time;
+      change = true;
+    }
+
+  if (device->focus_implicit_frame == f)
+    {
+      device->focus_implicit_time = time;
+      change = true;
+    }
+
+  /* If F isn't currently focused, update the focus state.  */
+  if (change && f != dpyinfo->x_focus_frame)
+    xi_handle_focus_change (dpyinfo);
+}
+
 #endif
 
 /* The focus may have changed.  Figure out if it is a real focus change,
@@ -21285,6 +21315,16 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 		    }
 		}
 
+	      if (f)
+		{
+		  /* If the user interacts with a frame that's focused
+		     on another device, but not the current focus
+		     frame, make it the focus frame.  */
+		  if (device)
+		    xi_handle_interaction (dpyinfo, f, device,
+					   xev->time);
+		}
+
 #ifdef USE_GTK
 	      if (!f)
 		{
@@ -21566,6 +21606,16 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 
 	      f = x_any_window_to_frame (dpyinfo, xev->event);
 
+	      if (f)
+		{
+		  /* If the user interacts with a frame that's focused
+		     on another device, but not the current focus
+		     frame, make it the focus frame.  */
+		  if (device)
+		    xi_handle_interaction (dpyinfo, f, device,
+					   xev->time);
+		}
+
 	      XKeyPressedEvent xkey;
 
 	      memset (&xkey, 0, sizeof xkey);
-- 
2.39.2