patch 8.2.0851: can't distinguish <M-a> from accented "a" in the GUI

Problem:    Can't distinguish <M-a> from accented "a" in the GUI.
Solution:   Use another way to make mapping <C-bslash> work. (closes #6163)

src/getchar.c

@@ -1593,17 +1593,27 @@ merge_modifyOtherKeys(int c_arg)
     if (mod_mask & MOD_MASK_CTRL)
     {
 	if ((c >= '`' && c <= 0x7f) || (c >= '@' && c <= '_'))
-	{
 	    c &= 0x1f;
-	    mod_mask &= ~MOD_MASK_CTRL;
-	}
 	else if (c == '6')
-	{
 	    // CTRL-6 is equivalent to CTRL-^
 	    c = 0x1e;
+#ifdef FEAT_GUI_GTK
+	// These mappings look arbitrary at the first glance, but in fact
+	// resemble quite exactly the behaviour of the GTK+ 1.2 GUI on my
+	// machine.  The only difference is BS vs. DEL for CTRL-8 (makes
+	// more sense and is consistent with usual terminal behaviour).
+	else if (c == '2')
+	    c = NUL;
+	else if (c >= '3' && c <= '7')
+	    c = c ^ 0x28;
+	else if (c == '8')
+	    c = BS;
+	else if (c == '?')
+	    c = DEL;
+#endif
+	if (c != c_arg)
 	    mod_mask &= ~MOD_MASK_CTRL;
     }
-    }
     if ((mod_mask & (MOD_MASK_META | MOD_MASK_ALT))
 	    && c >= 0 && c <= 127)
     {

src/gui.c

@@ -803,8 +803,8 @@ gui_init(void)
 	if (!p_beval)
 	    gui_mch_disable_beval_area(balloonEval);
 #endif
-	// In case the terminal was used before ":gui".
+	// In the GUI modifiers are prepended to keys.
-	seenModifyOtherKeys = FALSE;
+	seenModifyOtherKeys = TRUE;
 
 #if defined(FEAT_XIM) && defined(FEAT_GUI_GTK)
 	if (!im_xim_isvalid_imactivate())

src/gui_gtk_x11.c

@@ -1018,51 +1018,22 @@ focus_out_event(GtkWidget *widget UNUSED,
  * The output is written to string, which must have room for at least 6 bytes
  * plus the NUL terminator.  Returns the length in bytes.
  *
- * This function is used in the GTK+ 2 GUI only.  The GTK+ 1 code makes use
+ * event->string is evil; see here why:
- * of GdkEventKey::string instead.  But event->string is evil; see here why:
  * http://developer.gnome.org/doc/API/2.0/gdk/gdk-Event-Structures.html#GdkEventKey
  */
     static int
-keyval_to_string(unsigned int keyval, unsigned int *state, char_u *string)
+keyval_to_string(unsigned int keyval, char_u *string)
 {
     int	    len;
     guint32 uc;
 
     uc = gdk_keyval_to_unicode(keyval);
     if (uc != 0)
-    {
-	// Check for CTRL-char
-	if ((*state & GDK_CONTROL_MASK) && uc >= 0x20 && uc < 0x80)
-	{
-	    // These mappings look arbitrary at the first glance, but in fact
-	    // resemble quite exactly the behaviour of the GTK+ 1.2 GUI on my
-	    // machine.  The only difference is BS vs. DEL for CTRL-8 (makes
-	    // more sense and is consistent with usual terminal behaviour).
-	    if (uc >= '@')
-		string[0] = uc & 0x1F;
-	    else if (uc == '2')
-		string[0] = NUL;
-	    else if (uc >= '3' && uc <= '7')
-		string[0] = uc ^ 0x28;
-	    else if (uc == '8')
-		string[0] = BS;
-	    else if (uc == '?')
-		string[0] = DEL;
-	    else
-		string[0] = uc;
-	    len = 1;
-
-	    if (string[0] != uc)
-		// The modifier was used, remove it.
-		*state = *state & ~GDK_CONTROL_MASK;
-	}
-	else
     {
 	// Translate a normal key to UTF-8.  This doesn't work for dead
 	// keys of course, you _have_ to use an input method for that.
 	len = utf_char2bytes((int)uc, string);
     }
-    }
     else
     {
 	// Translate keys which are represented by ASCII control codes in Vim.
@@ -1173,7 +1144,7 @@ key_press_event(GtkWidget *widget UNUSED,
     else
 #endif
     {
-	len = keyval_to_string(key_sym, &state, string2);
+	len = keyval_to_string(key_sym, string2);
 
 	// Careful: convert_input() doesn't handle the NUL character.
 	// No need to convert pure ASCII anyway, thus the len > 1 check.
@@ -1261,6 +1232,9 @@ key_press_event(GtkWidget *widget UNUSED,
     }
     else
     {
+	// <C-H> and <C-h> mean the same thing, always use "H"
+	if ((modifiers & MOD_MASK_CTRL) && ASCII_ISALPHA(key))
+	    key = TOUPPER_ASC(key);
 	string[0] = key;
 	len = 1;
     }

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    851,
 /**/
     850,
 /**/