bug#65068: 29.1; xkb-interception interaction causes problems with key combinations

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Alexander Prähauser <alexander.praehauser@gmx.at> writes:

> The significance of the U-key is that when I press Space-CapsLock-U
> (or any other key, I think), it doesn't send the signal
> that is configured by xkb but the corresponding key of the default
> English keymap (u for the U-key), which really
> shouldn't happen. Anyway, here the output after pressing the sequence
> you wanted:
>
> keycode: 48, keysym: 65027, 0
> keycode: 192, keysym: 269025093, 0
> keycode: 37, keysym: 65507, 0
> keycode: 37, keysym: 65507, 0 <- Control_R
> keycode: 48, keysym: 65027, 0 <- AC11, ISO_Level3_Shift
> keycode: 192, keysym: 269025093, <- XF86Launch5

This attests to Emacs registering the keysyms you meant it to.  If
ISO_Level3_Shift is registered as a modifier key, it should not be
translated into keyboard input afterwards, let alone the original
apostrophe symbol.

Does this patch resolve your problems with typing level 3 characters
coupled with Control?

diff --git a/src/xterm.c b/src/xterm.c
index 6a1642ff56e..7391041ea0c 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -23734,6 +23734,7 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 	      ptrdiff_t i;
 	      unsigned int old_state;
 	      struct xi_device_t *device, *source;
+	      bool is_modifier_key;
 
 	      coding = Qlatin_1;
 
@@ -24175,17 +24176,7 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 		       /* Any "vendor-specific" key is ok.  */
 		       || (keysym & (1 << 28))
 		       || (keysym != NoSymbol && nbytes == 0))
-		      && ! (IsModifierKey (keysym)
-			    /* The symbols from XK_ISO_Lock
-			       to XK_ISO_Last_Group_Lock
-			       don't have real modifiers but
-			       should be treated similarly to
-			       Mode_switch by Emacs. */
-#if defined XK_ISO_Lock && defined XK_ISO_Last_Group_Lock
-			    || (XK_ISO_Lock <= keysym
-				&& keysym <= XK_ISO_Last_Group_Lock)
-#endif
-			    ))
+		      && !is_modifier_key)
 		    {
 		      STORE_KEYSYM_FOR_DEBUG (keysym);
 		      /* make_lispy_event will convert this to a symbolic
@@ -24204,7 +24195,11 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 		      STORE_KEYSYM_FOR_DEBUG (copy_bufptr[i]);
 		    }
 
-		  if (nbytes)
+		  /* Mind that NBYTES can be set even if KEYSYM
+		     represents a modifier key, but that no character
+		     events should be sent in that case.  */
+
+		  if (nbytes && !is_modifier_key)
 		    {
 		      inev.ie.kind = MULTIBYTE_CHAR_KEYSTROKE_EVENT;
 		      inev.ie.arg = make_unibyte_string (copy_bufptr, nbytes);