Win32: Implement text input focus with IMM

When explicit text input focus is inactive, keep GLFW from processing its
own WM_IME_* composition, preedit, candidate and IME status paths while
preserving normal key input.

On Win32, gating GLFW message handling is not enough to prevent native IMM
candidate UI from appearing. Temporarily associate a NULL HIMC with the
window while explicit text input focus is out, then restore the saved HIMC
when focus returns.

Cancel and clear active preedit/candidate state as focus-out cleanup. After
restoring the HIMC, reapply the preedit cursor rectangle so candidate UI
uses the latest location.

src/win32_init.c

@@ -172,6 +172,8 @@ static GLFWbool loadLibraries(void)
             _glfwPlatformGetModuleSymbol(_glfw.win32.imm32.instance, "ImmGetCompositionStringW");
         _glfw.win32.imm32.ImmGetContext_ = (PFN_ImmGetContext)
             _glfwPlatformGetModuleSymbol(_glfw.win32.imm32.instance, "ImmGetContext");
+        _glfw.win32.imm32.ImmAssociateContext_ = (PFN_ImmAssociateContext)
+            _glfwPlatformGetModuleSymbol(_glfw.win32.imm32.instance, "ImmAssociateContext");
         _glfw.win32.imm32.ImmGetConversionStatus_ = (PFN_ImmGetConversionStatus)
             _glfwPlatformGetModuleSymbol(_glfw.win32.imm32.instance, "ImmGetConversionStatus");
         _glfw.win32.imm32.ImmGetDescriptionW_ = (PFN_ImmGetDescriptionW)

src/win32_platform.h

@@ -263,6 +263,7 @@ typedef LONG (WINAPI * PFN_RtlVerifyVersionInfo)(OSVERSIONINFOEXW*,ULONG,ULONGLO
 typedef DWORD  (WINAPI * PFN_ImmGetCandidateListW)(HIMC,DWORD,LPCANDIDATELIST,DWORD);
 typedef LONG (WINAPI * PFN_ImmGetCompositionStringW)(HIMC,DWORD,LPVOID,DWORD);
 typedef HIMC (WINAPI * PFN_ImmGetContext)(HWND);
+typedef HIMC (WINAPI * PFN_ImmAssociateContext)(HWND,HIMC);
 typedef BOOL (WINAPI * PFN_ImmGetConversionStatus)(HIMC,LPDWORD,LPDWORD);
 typedef UINT (WINAPI * PFN_ImmGetDescriptionW)(HKL,LPWSTR,UINT);
 typedef BOOL (WINAPI * PFN_ImmGetOpenStatus)(HIMC);
@@ -274,6 +275,7 @@ typedef BOOL (WINAPI * PFN_ImmSetOpenStatus)(HIMC,BOOL);
 #define ImmGetCandidateListW _glfw.win32.imm32.ImmGetCandidateListW_
 #define ImmGetCompositionStringW _glfw.win32.imm32.ImmGetCompositionStringW_
 #define ImmGetContext _glfw.win32.imm32.ImmGetContext_
+#define ImmAssociateContext _glfw.win32.imm32.ImmAssociateContext_
 #define ImmGetConversionStatus _glfw.win32.imm32.ImmGetConversionStatus_
 #define ImmGetDescriptionW _glfw.win32.imm32.ImmGetDescriptionW_
 #define ImmGetOpenStatus _glfw.win32.imm32.ImmGetOpenStatus_
@@ -380,6 +382,7 @@ typedef struct _GLFWlibraryWGL
 typedef struct _GLFWwindowWin32
 {
     HWND                handle;
+    HIMC                textInputContext;
     HICON               bigIcon;
     HICON               smallIcon;
 
@@ -473,6 +476,7 @@ typedef struct _GLFWlibraryWin32
         PFN_ImmGetCandidateListW        ImmGetCandidateListW_;
         PFN_ImmGetCompositionStringW    ImmGetCompositionStringW_;
         PFN_ImmGetContext               ImmGetContext_;
+        PFN_ImmAssociateContext         ImmAssociateContext_;
         PFN_ImmGetConversionStatus      ImmGetConversionStatus_;
         PFN_ImmGetDescriptionW          ImmGetDescriptionW_;
         PFN_ImmGetOpenStatus            ImmGetOpenStatus_;

src/win32_window.c

@@ -834,6 +834,11 @@ static void clearImmPreedit(_GLFWwindow* window)
     _glfwInputPreedit(window);
 }
 
+static GLFWbool textInputFocusDisabled(_GLFWwindow* window)
+{
+    return window->textInputFocusInitialized && !window->textInputFocus;
+}
+
 // Commit the result texts of Imm32 to character-callback
 //
 static GLFWbool commitImmResultStr(_GLFWwindow* window)
@@ -904,6 +909,9 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
     {
         case WM_IME_SETCONTEXT:
         {
+            if (textInputFocusDisabled(window))
+                break;
+
             // To draw preedit text by an application side
             if (lParam & ISC_SHOWUICOMPOSITIONWINDOW)
                 lParam &= ~ISC_SHOWUICOMPOSITIONWINDOW;
@@ -1146,6 +1154,9 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
 
         case WM_IME_COMPOSITION:
         {
+            if (textInputFocusDisabled(window))
+                return 0;
+
             if (lParam & (GCS_RESULTSTR | GCS_COMPSTR))
             {
                 if (lParam & GCS_RESULTSTR)
@@ -1159,6 +1170,9 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
 
         case WM_IME_ENDCOMPOSITION:
         {
+            if (textInputFocusDisabled(window))
+                return 0;
+
             clearImmPreedit(window);
             // Usually clearing candidates in IMN_CLOSECANDIDATE is sufficient.
             // However, some IME need it here, e.g. Google Japanese Input.
@@ -1168,6 +1182,9 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
 
         case WM_IME_NOTIFY:
         {
+            if (textInputFocusDisabled(window))
+                return 0;
+
             switch (wParam)
             {
                 case IMN_SETOPENSTATUS:
@@ -1955,6 +1972,9 @@ void _glfwDestroyWindowWin32(_GLFWwindow* window)
 
     if (window->win32.handle)
     {
+        if (window->win32.textInputContext)
+            ImmAssociateContext(window->win32.handle, window->win32.textInputContext);
+
         RemovePropW(window->win32.handle, L"GLFW");
         DestroyWindow(window->win32.handle);
         window->win32.handle = NULL;
@@ -2857,6 +2877,9 @@ void _glfwUpdatePreeditCursorRectangleWin32(_GLFWwindow* window)
     int h = preedit->cursorHeight;
 
     COMPOSITIONFORM areaRect = { CFS_RECT, { x, y }, { x, y, x + w, y + h } };
+    if (!hIMC)
+        return;
+
     ImmSetCompositionWindow(hIMC, &areaRect);
 
     CANDIDATEFORM excludeRect = { 0, CFS_EXCLUDE, { x, y }, { x, y, x + w, y + h } };
@@ -2869,29 +2892,91 @@ void _glfwResetPreeditTextWin32(_GLFWwindow* window)
 {
     HWND hWnd = window->win32.handle;
     HIMC hIMC = ImmGetContext(hWnd);
-    ImmNotifyIME(hIMC, NI_COMPOSITIONSTR, CPS_CANCEL, 0);
-    ImmReleaseContext(hWnd, hIMC);
+    GLFWbool releaseContext = GLFW_TRUE;
+
+    if (!hIMC && window->win32.textInputContext)
+    {
+        hIMC = window->win32.textInputContext;
+        releaseContext = GLFW_FALSE;
+    }
+
+    if (hIMC)
+    {
+        ImmNotifyIME(hIMC, NI_COMPOSITIONSTR, CPS_CANCEL, 0);
+        if (releaseContext)
+            ImmReleaseContext(hWnd, hIMC);
+    }
+
+    clearImmPreedit(window);
+    clearImmCandidate(window);
 }
 
 void _glfwSetTextInputFocusWin32(_GLFWwindow* window, GLFWbool focused)
 {
-    // TODO: Add safe IMM/TSF text input focus plumbing without changing IME status.
+    if (focused)
+    {
+        if (window->win32.textInputContext)
+        {
+            ImmAssociateContext(window->win32.handle,
+                                window->win32.textInputContext);
+            window->win32.textInputContext = NULL;
+            _glfwUpdatePreeditCursorRectangleWin32(window);
+        }
+    }
+    else
+    {
+        _glfwResetPreeditTextWin32(window);
+
+        if (!window->win32.textInputContext)
+        {
+            window->win32.textInputContext =
+                ImmAssociateContext(window->win32.handle, NULL);
+        }
+    }
 }
 
 void _glfwSetIMEStatusWin32(_GLFWwindow* window, int active)
 {
     HWND hWnd = window->win32.handle;
-    HIMC hIMC = ImmGetContext(hWnd);
+    HIMC hIMC = window->win32.textInputContext;
+    GLFWbool releaseContext = GLFW_FALSE;
+
+    if (!hIMC)
+    {
+        hIMC = ImmGetContext(hWnd);
+        releaseContext = GLFW_TRUE;
+    }
+
+    if (!hIMC)
+        return;
+
     ImmSetOpenStatus(hIMC, active ? TRUE : FALSE);
-    ImmReleaseContext(hWnd, hIMC);
+
+    if (releaseContext)
+        ImmReleaseContext(hWnd, hIMC);
 }
 
 int _glfwGetIMEStatusWin32(_GLFWwindow* window)
 {
     HWND hWnd = window->win32.handle;
-    HIMC hIMC = ImmGetContext(hWnd);
-    BOOL result = ImmGetOpenStatus(hIMC);
-    ImmReleaseContext(hWnd, hIMC);
+    HIMC hIMC = window->win32.textInputContext;
+    GLFWbool releaseContext = GLFW_FALSE;
+    BOOL result;
+
+    if (!hIMC)
+    {
+        hIMC = ImmGetContext(hWnd);
+        releaseContext = GLFW_TRUE;
+    }
+
+    if (!hIMC)
+        return GLFW_FALSE;
+
+    result = ImmGetOpenStatus(hIMC);
+
+    if (releaseContext)
+        ImmReleaseContext(hWnd, hIMC);
+
     return result ? GLFW_TRUE : GLFW_FALSE;
 }