Add an option to rely on passive touch grabs on X

This prevents crashes with three-finger touch gestures on
Xwayland running on GNOME Shell, and also when running some
programs that make use of an Emacs window's XID on the same X
server.

* src/xterm.c (handle_one_xevent): Respect
`x-input-grab-touch-events'.
(syms_of_xterm): New option `x-input-grab-touch-events.

1 files changed, 14 insertions, 4 deletions

diff --git a/src/xterm.c b/src/xterm.c
index 9b4bd4b8db2..6a05ad98949 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -11134,8 +11134,9 @@ handle_one_xevent (struct x_display_info *dpyinfo,
                     {
                       *finish = X_EVENT_DROP;
                       x_catch_errors (dpyinfo->display);
-                      XIAllowTouchEvents (dpyinfo->display, xev->deviceid,
-                                          xev->detail, xev->event, XIAcceptTouch);
+                      if (x_input_grab_touch_events)
+                        XIAllowTouchEvents (dpyinfo->display, xev->deviceid,
+                                            xev->detail, xev->event, XIAcceptTouch);
                       if (!x_had_errors_p (dpyinfo->display))
                         {
                           xi_link_touch_point (device, xev->detail, xev->event_x,
@@ -11154,8 +11155,9 @@ handle_one_xevent (struct x_display_info *dpyinfo,
                   else
                     {
                       x_catch_errors (dpyinfo->display);
-                      XIAllowTouchEvents (dpyinfo->display, xev->deviceid,
-                                          xev->detail, xev->event, XIRejectTouch);
+                      if (x_input_grab_touch_events)
+                        XIAllowTouchEvents (dpyinfo->display, xev->deviceid,
+                                            xev->detail, xev->event, XIRejectTouch);
                       x_uncatch_errors ();
                     }
 #endif
@@ -15946,4 +15948,12 @@ support. */);
 This provides better support for some modern input methods, and is
 only effective when Emacs is built with GTK.  */);
   x_gtk_use_native_input = false;
+
+  DEFVAR_BOOL ("x-input-grab-touch-events", x_input_grab_touch_events,
+               doc: /* Non-nil means to actively grab touch events.
+This means touch sequences that started on an Emacs frame will
+reliably continue to receive updates even if the finger moves off the
+frame, but may cause crashes with some window managers and/or external
+programs.  */);
+  x_input_grab_touch_events = true;
 }