From bdc8314235a73ef8b6079efdef00bdfdd39c13ca Mon Sep 17 00:00:00 2001
From: Shun-ichi Goto <shunichi.goto@gmail.com>
Date: Wed, 24 Jun 2026 19:30:36 +0900
Subject: [PATCH] Respect IME preedit cursor position on Win32 (#21632)

---
 .../Avalonia.Win32/Input/Imm32InputMethod.cs  | 80 +++++++++++++++----
 1 file changed, 63 insertions(+), 17 deletions(-)

diff --git a/src/Windows/Avalonia.Win32/Input/Imm32InputMethod.cs b/src/Windows/Avalonia.Win32/Input/Imm32InputMethod.cs
index 65827607c1..588e7a134b 100644
--- a/src/Windows/Avalonia.Win32/Input/Imm32InputMethod.cs
+++ b/src/Windows/Avalonia.Win32/Input/Imm32InputMethod.cs
@@ -25,6 +25,7 @@ namespace Avalonia.Win32.Input
         private const int CaretMargin = 1;
 
         private bool _ignoreComposition;
+        private int? _compositionCursorPosition;
 
         public TextInputMethodClient? Client { get; private set; }
 
@@ -110,6 +111,7 @@ namespace Avalonia.Win32.Input
             _langId = 0;
 
             IsComposing = false;
+            _compositionCursorPosition = null;
         }
 
         //Dependant on CurrentThread. When Avalonia will support Multiple Dispatchers -
@@ -118,6 +120,8 @@ namespace Avalonia.Win32.Input
 
         public void Reset()
         {
+            _compositionCursorPosition = null;
+
             Dispatcher.UIThread.Post(() =>
             {
                 var himc = ImmGetContext(Hwnd);
@@ -141,6 +145,7 @@ namespace Avalonia.Win32.Input
                     IsComposing = false;
 
                     Composition = null;
+                    _compositionCursorPosition = null;
                 }
             });
         }
@@ -150,8 +155,9 @@ namespace Avalonia.Win32.Input
             if(Client != null)
             {
                 Composition = null;
+                _compositionCursorPosition = null;
 
-                Client.SetPreeditText(null);
+                Client.SetPreeditText(null, null);
             }
 
             Client = client;
@@ -266,16 +272,17 @@ namespace Avalonia.Win32.Input
             // we're skipping this. not usable on windows
         }
 
-        public void CompositionChanged(string? composition)
+        public void CompositionChanged(string? composition, int? cursorPosition)
         {
             Composition = composition;
+            _compositionCursorPosition = cursorPosition;
 
             if (!IsActive || !Client.SupportsPreedit)
             {
                 return;
             }
 
-            Client.SetPreeditText(composition);
+            Client.SetPreeditText(composition, cursorPosition);
         }
         
         public string? GetCompositionString(GCS flag)
@@ -290,13 +297,40 @@ namespace Avalonia.Win32.Input
             return ImmGetCompositionString(himc, flag);
         }
 
+        private int? GetCompositionCursorPosition()
+        {
+            if (!IsComposing)
+            {
+                return null;
+            }
+
+            var himc = ImmGetContext(Hwnd);
+
+            if (himc == IntPtr.Zero)
+            {
+                return null;
+            }
+
+            try
+            {
+                var cursorPosition = ImmGetCompositionString(himc, GCS.GCS_CURSORPOS, IntPtr.Zero, 0);
+
+                return cursorPosition >= 0 ? cursorPosition : null;
+            }
+            finally
+            {
+                ImmReleaseContext(Hwnd, himc);
+            }
+        }
+
         public void HandleCompositionStart()
         {
             Composition = null;
+            _compositionCursorPosition = null;
 
             if (IsActive)
             {
-                Client.SetPreeditText(null);
+                Client.SetPreeditText(null, null);
 
                 if (Client.SupportsSurroundingText && Client.Selection.Start != Client.Selection.End)
                 {
@@ -325,10 +359,11 @@ namespace Avalonia.Win32.Input
             }
 
             Composition = null;
+            _compositionCursorPosition = null;
 
             if (IsActive)
             {
-                Client.SetPreeditText(null);
+                Client.SetPreeditText(null, null);
             }
         }
 
@@ -342,25 +377,27 @@ namespace Avalonia.Win32.Input
             }
 
             var flags = (GCS)ToInt32(lParam);
+            var resultChanged = (flags & GCS.GCS_RESULTSTR) != 0;
             
             if (flags == 0)
             {
-                CompositionChanged("");
+                CompositionChanged("", null);
             }
 
-            if ((flags & GCS.GCS_RESULTSTR) != 0)
+            if (resultChanged)
             {
                 var resultString = GetCompositionString(GCS.GCS_RESULTSTR);
 
-                if (_parent != null && !string.IsNullOrEmpty(resultString))
-                {
-                    Composition = null;
+                Composition = null;
+                _compositionCursorPosition = null;
 
-                    if (IsActive)
-                    {
-                        Client.SetPreeditText(null);
-                    }
+                if (IsActive)
+                {
+                    Client.SetPreeditText(null, null);
+                }
 
+                if (_parent != null && !string.IsNullOrEmpty(resultString))
+                {
                     var e = new RawTextInputEventArgs(WindowsKeyboardDevice.Instance, timestamp, _parent.Owner, resultString);
 
                     if (_parent.Input != null)
@@ -372,11 +409,20 @@ namespace Avalonia.Win32.Input
                 }
             }
 
-            if ((flags & GCS.GCS_COMPSTR) != 0)
+            var compositionChanged = (flags & GCS.GCS_COMPSTR) != 0;
+            var cursorPositionChanged = (flags & GCS.GCS_CURSORPOS) != 0;
+
+            if (compositionChanged || (cursorPositionChanged && !resultChanged))
             {
-                var compositionString = GetCompositionString(GCS.GCS_COMPSTR);
+                var compositionString = compositionChanged
+                    ? GetCompositionString(GCS.GCS_COMPSTR)
+                    : Composition;
+
+                var cursorPosition = cursorPositionChanged
+                    ? GetCompositionCursorPosition()
+                    : _compositionCursorPosition;
 
-                CompositionChanged(compositionString);
+                CompositionChanged(compositionString, cursorPosition);
             }
         }