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

From a0b062b1aa876fee11a3c20cd68055bfbfff4bc6 Mon Sep 17 00:00:00 2001
From: zdn <[email protected]>
Date: Thu, 30 Jul 2026 00:42:10 +0800
Subject: [PATCH] Implement preedit text rendering for the IME window on w32

* lisp/term/w32-win.el (w32-clear-preedit-text, w32-preedit-text):
Draw w32 IME preedit text.
* src/w32fns.c (w32_wnd_proc): Extend the WM_IME_COMPOSITION event
and handle edge cases in WM_IME_STARTCOMPOSITION
* src/w32term.c (w32_read_socket): Handle WM_IME_COMPOSITION message
and set preedit text parameters
---
 lisp/term/w32-win.el | 32 ++++++++++++++++
 src/w32fns.c         | 87 +++++++++++++++++++++++++++++++++++++++-----
 src/w32term.c        | 39 ++++++++++++++++++++
 3 files changed, 149 insertions(+), 9 deletions(-)

diff --git a/lisp/term/w32-win.el b/lisp/term/w32-win.el
index 385f488..c6d9e12 100644
--- a/lisp/term/w32-win.el
+++ b/lisp/term/w32-win.el
@@ -621,6 +621,38 @@ w32-find-non-USB-fonts
     (clear-font-cache)
     (and val (setq w32-non-USB-fonts val))))
 
+;; The preedit rendering code below is adapted from x-win.el,
+;; with the variable and function prefixes changed to w32 and a few minor modifications.
+(defvar w32-preedit-overlay nil
+  "The overlay currently used to display preedit text from a compose sequence.")
+
+(defun w32-clear-preedit-text ()
+  "Clear the pre-edit overlay and remove itself from `pre-command-hook'.
+This function should be installed in `pre-command-hook' whenever
+preedit text is displayed."
+  (when w32-preedit-overlay
+    (delete-overlay w32-preedit-overlay)
+    (setq w32-preedit-overlay nil))
+  (remove-hook 'pre-command-hook #'w32-clear-preedit-text))
+
+(defun w32-preedit-text (event)
+  "Display preedit text from a compose sequence in EVENT.
+EVENT is a preedit-text event."
+  (interactive "e")
+  (when w32-ime-preedit
+    (when w32-preedit-overlay
+      (delete-overlay w32-preedit-overlay)
+      (setq w32-preedit-overlay nil)
+      (remove-hook 'pre-command-hook #'w32-clear-preedit-text))
+    (when (nth 1 event)
+      (let ((string (propertize (nth 1 event) 'face '(:underline t))))
+        (setq w32-preedit-overlay (make-overlay (point) (point)))
+        (add-hook 'pre-command-hook #'w32-clear-preedit-text)
+        (overlay-put w32-preedit-overlay 'window (selected-window))
+        (overlay-put w32-preedit-overlay 'before-string string)))))
+
+(define-key special-event-map [preedit-text] 'w32-preedit-text)
+
 (provide 'w32-win)
 (provide 'term/w32-win)
 
diff --git a/src/w32fns.c b/src/w32fns.c
index 726fe52..5b41a33 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -271,6 +271,15 @@ #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.  */
+static wchar_t *composition_ime_data;
+
+/* Used to preserve the position of the IME window during the WM_IME_COMPOSITION event,
+   preventing other events from updating the w32_system_caret_x variable and
+   causing the IME window's x position to jitter,
+   in order to remain consistent with Windows' default behavior.  */
+static int composition_ime_pt_x = -1;
+
 /* W95 mousewheel handler */
 extern unsigned int msh_mousewheel;
 unsigned int msh_mousewheel = 0;
@@ -5058,15 +5067,44 @@ 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)
+	    {
+	      if (composition_ime_pt_x > 0)
+		form.ptCurrentPos.x = composition_ime_pt_x;
+	      /* 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 -99999 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 = -99999;
+	      form.rcArea.top = -99999;
+	    }
+	  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 +5126,37 @@ 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)
+	{
+	  composition_ime_pt_x = -1;
+	  my_post_msg (&wmsg, hwnd, msg, (WPARAM) "", 0);
+	  free (composition_ime_data);
+	}
+      else if (lParam & GCS_COMPSTR)
+	{
+	  if (composition_ime_pt_x < 0)
+	    composition_ime_pt_x = w32_system_caret_x;
+
+	  int size;
+	  HIMC context = get_ime_context_fn (hwnd);
+
+	  size = get_composition_string_fn (context, GCS_COMPSTR, NULL, 0);
+	  composition_ime_data = malloc (size);
+	  size = get_composition_string_fn (context, GCS_COMPSTR,
+					    composition_ime_data, size);
+	  release_ime_context_fn (hwnd, context);
+
+	  signal_user_input ();
+	  my_post_msg (&wmsg, hwnd, msg,
+		       (WPARAM) composition_ime_data, size);
+	}
+
+      goto dflt;
+
     case WM_IME_ENDCOMPOSITION:
       ignore_ime_char = 0;
       goto dflt;
diff --git a/src/w32term.c b/src/w32term.c
index 728f6ce..8fae547 100644
--- a/src/w32term.c
+++ b/src/w32term.c
@@ -5387,6 +5387,38 @@ w32_read_socket (struct terminal *terminal,
 	    }
 	  break;
 
+	case WM_IME_COMPOSITION:
+	  {
+	    f = w32_window_to_frame (dpyinfo, msg.msg.hwnd);
+	    Lisp_Object str = Qnil;
+	    int size = (int) msg.msg.lParam;
+
+	    if (size > 0)
+	      {
+		int utf8_len;
+		char *utf8_buf;
+		wchar_t *wbuf = (wchar_t *) msg.msg.wParam;
+		utf8_len = WideCharToMultiByte (CP_UTF8, 0,
+						wbuf, size / sizeof(wchar_t),
+						NULL, 0,
+						NULL, NULL);
+		utf8_buf = alloca (utf8_len);
+		utf8_len = WideCharToMultiByte(CP_UTF8, 0,
+					       wbuf, size / sizeof(wchar_t),
+					       utf8_buf, utf8_len,
+					       NULL, NULL);
+
+		str = make_string_from_utf8 (utf8_buf, utf8_len);
+	      }
+
+	    inev.kind = PREEDIT_TEXT_EVENT;
+	    inev.arg = str;
+	    inev.modifiers = msg.dwModifiers;
+	    XSETFRAME (inev.frame_or_window, f);
+	    inev.timestamp = msg.msg.time;
+	  }
+	  break;
+
         case WM_APPCOMMAND:
 	  f = w32_window_to_frame (dpyinfo, msg.msg.hwnd);
 
@@ -8392,6 +8424,13 @@ 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;
+
   /* Tell Emacs about this window system.  */
   Fprovide (Qw32, Qnil);
 }
-- 
2.54.0