bug#81506: [PATCH] Extend w32 window events to expose IME composition string interface

From f5465c81c95364a40aeb7dff16669280f08a9dbb Mon Sep 17 00:00:00 2001
From: zdn <[email protected]>
Date: Tue, 28 Jul 2026 08:40:32 +0800
Subject: [PATCH] Extend w32 window events to expose IME composition string
 interface

Add WM_IME_COMPOSITION event processing

Handle the composition string from IME and hide the
composition string window.  Add an Elisp hook to deliver the
composition result to Lisp code.

* src/w32fns.c (w32_wnd_proc): Hide composition string window.
* src/w32term.c (w32_read_socket): Read composition result to Lisp code.
---
 src/w32fns.c  | 83 +++++++++++++++++++++++++++++++++++++++++++++------
 src/w32term.c | 25 ++++++++++++++++
 2 files changed, 99 insertions(+), 9 deletions(-)

diff --git a/src/w32fns.c b/src/w32fns.c
index 726fe52..6774a26 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -271,6 +271,13 @@ #define CCHDEVICENAME 32
 /* Flag to selectively ignore WM_IME_CHAR messages.  */
 static int ignore_ime_char = 0;
 
+/* Fixed buffer for receiving IME composition (preedit) string.
+   IME composition strings are typically short, so 64 characters
+   is sufficient for most cases. The buffer is sized to 2 * 64
+   bytes because each wide character (wchar_t) occupies 2 bytes
+   under UTF-16 encoding on Windows. */
+static char composition_ime_buf[2 * 64];
+
 /* W95 mousewheel handler */
 extern unsigned int msh_mousewheel;
 unsigned int msh_mousewheel = 0;
@@ -5058,15 +5065,41 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	  form.ptCurrentPos.x = w32_system_caret_x;
 	  form.ptCurrentPos.y = w32_system_caret_y;
 
-	  form.rcArea.left = WINDOW_TEXT_TO_FRAME_PIXEL_X (w, 0);
-	  form.rcArea.top = (WINDOW_TOP_EDGE_Y (w)
-			     + w32_system_caret_hdr_height);
-	  form.rcArea.right = (WINDOW_BOX_RIGHT_EDGE_X (w)
-			       - WINDOW_RIGHT_MARGIN_WIDTH (w)
-			       - WINDOW_RIGHT_FRINGE_WIDTH (w));
-	  form.rcArea.bottom = (WINDOW_BOTTOM_EDGE_Y (w)
-				- WINDOW_BOTTOM_DIVIDER_WIDTH (w)
-				- w32_system_caret_mode_height);
+	  if (w32_ime_preedit)
+	    {
+	      /* After hiding the preedit window, the original position is a bit too
+		 high. Subtracting one line height gives a better vertical balance. */
+	      form.ptCurrentPos.y += FRAME_LINE_HEIGHT (f);
+	      /* Setting rcArea to -9999 is to hide the preedit window.
+		 Based on the documentation description, in the WM_IME_SETCONTEXT
+		 event, removing the ISC_SHOWUICOMPOSITIONWINDOW flag from lParam
+		 does work. However, this approach has side effects — once the
+		 preedit window is hidden, dynamically toggling the w32_ime_preedit
+		 variable from Elisp cannot bring the original preedit window back.
+		 As a workaround, I resorted to setting the preedit window
+		 coordinates (to off-screen) instead.
+		 The following was my original code:
+
+		 case WM_IME_SETCONTEXT:
+		   if (w32_ime_preedit)
+		     lParam &= ~ISC_SHOWUICOMPOSITIONWINDOW;
+		   goto dflt;
+		 See https://learn.microsoft.com/en-us/windows/win32/intl/wm-ime-setcontext
+	      */
+	      form.rcArea.left = -9999;
+	    }
+	  else
+	    {
+	      form.rcArea.left = WINDOW_TEXT_TO_FRAME_PIXEL_X (w, 0);
+	      form.rcArea.top = (WINDOW_TOP_EDGE_Y (w)
+				 + w32_system_caret_hdr_height);
+	      form.rcArea.right = (WINDOW_BOX_RIGHT_EDGE_X (w)
+				   - WINDOW_RIGHT_MARGIN_WIDTH (w)
+				   - WINDOW_RIGHT_FRINGE_WIDTH (w));
+	      form.rcArea.bottom = (WINDOW_BOTTOM_EDGE_Y (w)
+				    - WINDOW_BOTTOM_DIVIDER_WIDTH (w)
+				    - w32_system_caret_mode_height);
+	    }
 
 	  /* Punt if the window was deleted behind our back.  */
 	  if (!BUFFERP (w->contents))
@@ -5088,6 +5121,38 @@ w32_wnd_proc (HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 	 supported by the input method...  */
       goto dflt;
 
+    case WM_IME_COMPOSITION:
+      if (!w32_ime_preedit)
+	goto dflt;
+
+      if (lParam & GCS_RESULTSTR)
+	{
+	  my_post_msg (&wmsg, hwnd, WM_IME_COMPOSITION, (WPARAM) "", 0);
+	}
+      else if (lParam & GCS_COMPSTR)
+	{
+	  int size, wlen;
+	  wchar_t *wbuf = alloca (sizeof(composition_ime_buf));
+	  HIMC context = get_ime_context_fn (hwnd);
+
+	  memset(composition_ime_buf, 0, sizeof(composition_ime_buf));
+	  size = get_composition_string_fn (context, GCS_COMPSTR,
+					    wbuf, sizeof (composition_ime_buf));
+	  release_ime_context_fn (hwnd, context);
+
+	  wlen = WideCharToMultiByte (CP_UTF8, 0,
+				      wbuf,
+				      size / sizeof (wchar_t),
+				      composition_ime_buf,
+				      sizeof (composition_ime_buf),
+				      NULL, NULL);
+	  signal_user_input ();
+	  my_post_msg (&wmsg, hwnd, WM_IME_COMPOSITION,
+		       (WPARAM) &composition_ime_buf, wlen);
+	}
+
+      goto dflt;
+
     case WM_IME_ENDCOMPOSITION:
       ignore_ime_char = 0;
       goto dflt;
diff --git a/src/w32term.c b/src/w32term.c
index 728f6ce..fd9a3c0 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -5387,6 +5387,15 @@ w32_read_socket (struct terminal *terminal,
 	    }
 	  break;
 
+	case WM_IME_COMPOSITION:
+	  {
+	    int len = (int) msg.msg.lParam;
+	    char *buf = (char *) msg.msg.wParam;
+	    Lisp_Object str = make_string_from_utf8 (buf, len);
+	    CALLN (Frun_hook_with_args, Qw32_ime_composition_hook, str);
+	  }
+	  break;
+
         case WM_APPCOMMAND:
 	  f = w32_window_to_frame (dpyinfo, msg.msg.hwnd);
 
@@ -8392,6 +8401,22 @@ syms_of_w32term (void)
 API.  */);
   w32_add_wrapped_menu_bar_lines = 1;
 
+  DEFVAR_BOOL ("w32-ime-preedit",
+	       w32_ime_preedit,
+	       doc: /* Non-nil means report IME preedit strings to
+`w32-ime-composition-hook'.  When nil, the IME preedit is not
+reported and the IME works as normal.  */);
+  w32_ime_preedit = 0;
+
+  DEFVAR_LISP ("w32-ime-composition-hook",
+	       Vw32_ime_composition_hook,
+	       doc: /* Hook run when the IME composition string changes.
+This hook is called during IME composition, before the composed text
+is finalized and inserted into the buffer.  The hook function receives
+one argument, the current composition string (as a multibyte string).  */);
+  Vw32_ime_composition_hook = Qnil;
+  DEFSYM (Qw32_ime_composition_hook, "w32-ime-composition-hook");
+
   /* Tell Emacs about this window system.  */
   Fprovide (Qw32, Qnil);
 }
-- 
2.54.0