From 614d8d6c6325ddec9bd5eea0fbfdd52f3c50fccb Mon Sep 17 00:00:00 2001
From: farblos <43711228+farblos@users.noreply.github.com>
Date: Mon, 4 Sep 2023 18:30:53 +0200
Subject: [PATCH] MaskUsedModifiers: remove bits used for XKB group index

According to section "Xkb State to Core Protocol State
Transformation" of the "The X Keyboard Extension: Library
Specification", the Xkb-aware server reports group index in bits
13 and 14 of XEvent.xkey.state or XEvent.xbutton.state.  This
interferes with FVWM's modifier handling as described in #861.
---
 fvwm/bindings.c |  2 +-
 libs/defaults.h | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fvwm/bindings.c b/fvwm/bindings.c
index cea7efdf5..76b74bb1e 100644
--- a/fvwm/bindings.c
+++ b/fvwm/bindings.c
@@ -600,7 +600,7 @@ void print_bindings(void)
 /* Removes all unused modifiers from in_modifiers */
 unsigned int MaskUsedModifiers(unsigned int in_modifiers)
 {
-	return in_modifiers & ~mods_unused;
+	return in_modifiers & ~mods_unused & ~DEFAULT_XKB_MODS_IGNORED;
 }
 
 unsigned int GetUnusedModifiers(void)
diff --git a/libs/defaults.h b/libs/defaults.h
index d9c871f4a..ac30a1079 100644
--- a/libs/defaults.h
+++ b/libs/defaults.h
@@ -38,6 +38,16 @@
         ((Button1MotionMask * ((1 << NUMBER_OF_MOUSE_BUTTONS) - 1)))
 #define DEFAULT_MODS_UNUSED                LockMask
 
+/* Default modifier bits to ignore in XEvent.xkey.state or XEvent.xbutton.state.
+ * According to section "Xkb State to Core Protocol State Transformation" of the
+ * "The X Keyboard Extension: Library Specification", the Xkb-aware server
+ * reports group index in bits 13 and 14, which interferes with FVWM's modifier
+ * handling as described in https://github.com/fvwmorg/fvwm3/issues/861. */
+#ifdef HAVE_X11_XKBLIB_H
+#define DEFAULT_XKB_MODS_IGNORED ((1 << 13) | (1 << 14))
+#else
+#define DEFAULT_XKB_MODS_IGNORED 0
+#endif
 
 /*
  * These values may be adjusted to fine tune the menu looks.