uxtheme: If the application class is already set then OpenThemeData() should fail.

dlls/uxtheme/msstyles.c

@@ -1076,8 +1076,11 @@ PTHEME_CLASS MSSTYLES_OpenThemeClass(LPCWSTR pszAppName, LPCWSTR pszClassList, U
 
         parse_app_class_name(buf, szAppName, szClassName);
         if (szAppName[0])
-            cls = MSSTYLES_FindClass(tfActiveTheme, szAppName, szClassName);
-        else
+        {
+            /* If the application class is already set then fail */
+            if (pszAppName) return NULL;
+            pszAppName = szAppName;
+        }
         cls = MSSTYLES_FindClass(tfActiveTheme, pszAppName, szClassName);
 
         /* Fall back to default class if the specified subclass is not found */
@@ -1088,8 +1091,11 @@ PTHEME_CLASS MSSTYLES_OpenThemeClass(LPCWSTR pszAppName, LPCWSTR pszClassList, U
     if(!cls && *start) {
         parse_app_class_name(start, szAppName, szClassName);
         if (szAppName[0])
-            cls = MSSTYLES_FindClass(tfActiveTheme, szAppName, szClassName);
-        else
+        {
+            /* If the application class is already set then fail */
+            if (pszAppName) return NULL;
+            pszAppName = szAppName;
+        }
         cls = MSSTYLES_FindClass(tfActiveTheme, pszAppName, szClassName);
 
         /* Fall back to default class if the specified subclass is not found */

dlls/uxtheme/tests/system.c

@@ -562,9 +562,7 @@ static void test_OpenThemeData(void)
     ok(hRes == S_OK, "Got unexpected hr %#lx.\n", hRes);
     SetLastError(0xdeadbeef);
     hTheme = OpenThemeData(hWnd, L"explorer::treeview");
-    todo_wine
     ok(!hTheme, "OpenThemeData() should fail\n");
-    todo_wine
     ok(GetLastError() == E_PROP_ID_UNSUPPORTED, "Got unexpected %#lx.\n", GetLastError());
     SetWindowTheme(hWnd, NULL, NULL);