Really fix extended frame synchronization

* src/xterm.c (XTframe_up_to_date): Make sure the extended
counter ends up even.

1 files changed, 13 insertions, 4 deletions

diff --git a/src/xterm.c b/src/xterm.c
index d3d8dc468cd..49fc2b1bb74 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -1848,6 +1848,12 @@ x_update_end (struct frame *f)
 static void
 XTframe_up_to_date (struct frame *f)
 {
+#ifdef HAVE_XSYNC
+  XSyncValue add;
+  XSyncValue current;
+  Bool overflow_p;
+#endif
+
   eassert (FRAME_X_P (f));
   block_input ();
   FRAME_MOUSE_UPDATE (f);
@@ -1867,12 +1873,15 @@ XTframe_up_to_date (struct frame *f)
   if (FRAME_X_OUTPUT (f)->ext_sync_end_pending_p
       && FRAME_X_EXTENDED_COUNTER (f) != None)
     {
-      XSyncValue add;
-      Bool overflow_p;
+      current = FRAME_X_OUTPUT (f)->current_extended_counter_value;
+
+      if (XSyncValueLow32 (current) % 2)
+        XSyncIntToValue (&add, 1);
+      else
+        XSyncIntToValue (&add, 2);
 
-      XSyncIntToValue (&add, 1);
       XSyncValueAdd (&FRAME_X_OUTPUT (f)->current_extended_counter_value,
-                     add, add, &overflow_p);
+                     current, add, &overflow_p);
 
       if (overflow_p)
         emacs_abort ();