imm32: Disable cross thread access for ImmSetCompositionString().

dlls/imm32/imm.c

@@ -2377,6 +2377,9 @@ BOOL WINAPI ImmSetCompositionStringA(
     if (!data)
         return FALSE;
 
+    if (IMM_IsCrossThreadAccess(NULL, hIMC))
+        return FALSE;
+
     if (!(dwIndex == SCS_SETSTR ||
           dwIndex == SCS_CHANGEATTR ||
           dwIndex == SCS_CHANGECLAUSE ||
@@ -2432,6 +2435,9 @@ BOOL WINAPI ImmSetCompositionStringW(
     if (!data)
         return FALSE;
 
+    if (IMM_IsCrossThreadAccess(NULL, hIMC))
+        return FALSE;
+
     if (!(dwIndex == SCS_SETSTR ||
           dwIndex == SCS_CHANGEATTR ||
           dwIndex == SCS_CHANGECLAUSE ||

dlls/imm32/tests/imm32.c

@@ -984,6 +984,14 @@ static void test_ImmThreads(void)
     rc = ImmSetCompositionFontA(threadinfo.u_himc, &lf);
     ok(rc == 0, "ImmSetCompositionFont should fail\n");
 
+    /* CompositionString */
+    rc = ImmSetCompositionStringA(himc, SCS_SETSTR, "a", 2, NULL, 0);
+    ok(rc, "failed.\n");
+    rc = ImmSetCompositionStringA(otherHimc, SCS_SETSTR, "a", 2, NULL, 0);
+    ok(!rc, "should fail.\n");
+    rc = ImmSetCompositionStringA(threadinfo.u_himc, SCS_SETSTR, "a", 2, NULL, 0);
+    ok(!rc, "should fail.\n");
+
     /* CompositionWindow */
     rc = ImmSetCompositionWindow(himc, &cf);
     ok(rc != 0, "ImmSetCompositionWindow failed\n");