Make ignoring modifiers on IME input optional

By default, ignore modifier keys on IME input, but add
a variable to get back old behavior.
* src/w32fns.c (syms_of_w32fns): New variable
w32-ignore-modifiers-on-IME-input.
(w32_wnd_proc): Use it to ignore modifier keys when IME input is
used.  (Bug#44641)

* etc/NEWS: Announce the change and the new variable.

2 files changed, 21 insertions, 2 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 61f9c0e1fe2..4855cd3b61d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -48,7 +48,7 @@ skip leading or trailing empty lines of the buffer.
 
 ** Tramp
 
-*** The user option 'tramp-completion-reread-directory-timeout' is made obsolete.
+*** The user option 'tramp-completion-reread-directory-timeout' is now obsolete.
 
 
 * New Modes and Packages in Emacs 27.2
@@ -62,6 +62,13 @@ skip leading or trailing empty lines of the buffer.
 
 * Changes in Emacs 27.2 on Non-Free Operating Systems
 
+** Emacs now ignores modifier keys when IME input is used.
+By default, pressing Ctrl, Shift, and Alt keys while using IME input
+will no longer apply the modifiers to the produced characters, as
+there are IMEs which use keys with modifiers to input some
+characters.  Customize the variable 'w32-ignore-modifiers-on-IME-input'
+to nil to get back the old behavior.
+
 
 * Installation Changes in Emacs 27.1
 
diff --git a/src/w32fns.c b/src/w32fns.c
index 96cf34430d4..fd13a958651 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -4576,7 +4576,10 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
           int size, i;
           W32Msg wmsg;
           HIMC context = get_ime_context_fn (hwnd);
-          wmsg.dwModifiers = 0;
+          wmsg.dwModifiers =
+            w32_ignore_modifiers_on_IME_input
+            ? 0
+            : w32_get_key_modifiers (wParam, lParam);
           /* Get buffer size.  */
           size = get_composition_string_fn (context, GCS_RESULTSTR, NULL, 0);
           buffer = alloca (size);
@@ -10697,6 +10700,15 @@ tip frame.  */);
                doc: /* Non-nil means don't display the abort dialog when aborting.  */);
   w32_disable_abort_dialog = 0;
 
+  DEFVAR_BOOL ("w32-ignore-modifiers-on-IME-input",
+               w32_ignore_modifiers_on_IME_input,
+               doc: /* Whether to ignore modifier keys when processing input with IME.
+Some MS-Windows input methods use modifier keys such as Ctrl or Alt to input
+characters, in which case applying the modifiers will change the input.
+The default value of this variable is therefore t, to ignore modifier
+keys when IME input is received.  */);
+  w32_ignore_modifiers_on_IME_input = true;
+
 #if 0 /* TODO: Port to W32 */
   defsubr (&Sx_change_window_property);
   defsubr (&Sx_delete_window_property);