[ROSTESTS] Add an interactive "dialog_initvisible" test (#9362)

CORE-20706

Its purpose is to observe whether or not a dialog that has the
`WS_VISIBLE` style explicitly set, briefly becomes visible
even if it gets terminated early from its `WM_INITDIALOG` handler.
(On Windows it does NOT become visible.)

modules/rostests/win32/user32/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_subdirectory(biditext)
+add_subdirectory(dialog_initvisible)
 add_subdirectory(editalign)
 add_subdirectory(menuaccels)
 add_subdirectory(messagebox)

modules/rostests/win32/user32/dialog_initvisible/CMakeLists.txt

@@ -0,0 +1,5 @@
+
+add_executable(dialog_initvisible dialog_initvisible.c dialog_initvisible.rc)
+set_module_type(dialog_initvisible win32gui UNICODE)
+add_importlibs(dialog_initvisible user32 msvcrt kernel32)
+add_rostests_file(TARGET dialog_initvisible SUBDIR suppl)

modules/rostests/win32/user32/dialog_initvisible/dialog_initvisible.c

@@ -0,0 +1,65 @@
+/*
+ * PROJECT:     ReactOS Tests
+ * LICENSE:     GPL-2.0-or-later (https://spdx.org/licenses/GPL-2.0-or-later)
+ * PURPOSE:     Tests whether a dialog is getting visible even if WM_INITDIALOG ends it.
+ * COPYRIGHT:   Copyright 2026 Hermès Bélusca-Maïto <hermes.belusca-maito@reactos.org>
+ */
+
+#define WIN32_NO_STATUS
+#include <windef.h>
+#include <winbase.h>
+#include <winuser.h>
+
+#define NTOS_MODE_USER
+#include <ndk/iotypes.h> // Needed for kdtypes.h
+#include <ndk/ketypes.h> //   "     "     "
+#include <ndk/kdtypes.h> // For SharedUserData
+
+#include "resource.h"
+
+INT_PTR CALLBACK
+AboutDlgProc(
+    _In_ HWND hDlg,
+    _In_ UINT uMsg,
+    _In_ WPARAM wParam,
+    _In_ LPARAM lParam)
+{
+    UNREFERENCED_PARAMETER(lParam);
+    switch (uMsg)
+    {
+        case WM_INITDIALOG:
+        {
+            /* Intentional breakpoint for you to step-by-step debug into a debugger! */
+            if (IsDebuggerPresent() || SharedUserData->KdDebuggerEnabled)
+                __debugbreak();
+
+            SendMessageW(hDlg, WM_COMMAND, MAKEWPARAM(IDOK, BN_CLICKED), 0);
+            EndDialog(hDlg, -1);
+            return (INT_PTR)TRUE;
+        }
+
+        case WM_COMMAND:
+        {
+            if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
+            {
+                EndDialog(hDlg, LOWORD(wParam));
+                return (INT_PTR)TRUE;
+            }
+            break;
+        }
+    }
+    return (INT_PTR)FALSE;
+}
+
+int APIENTRY
+wWinMain(
+    _In_ HINSTANCE hInstance,
+    _In_ HINSTANCE hPrevInstance,
+    _In_ LPWSTR lpCmdLine,
+    _In_ int nCmdShow)
+{
+    UNREFERENCED_PARAMETER(hPrevInstance);
+    UNREFERENCED_PARAMETER(lpCmdLine);
+
+    return DialogBoxW(hInstance, MAKEINTRESOURCEW(IDD_ABOUTBOX), NULL, AboutDlgProc);
+}

modules/rostests/win32/user32/dialog_initvisible/dialog_initvisible.rc

@@ -0,0 +1,17 @@
+
+#include <winnt.rh>
+#include <winuser.rh>
+#include "resource.h"
+
+LANGUAGE LANG_NEUTRAL, SUBLANG_NEUTRAL
+
+// NOTE: Ensure the WS_VISIBLE flag is set for the test.
+IDD_ABOUTBOX DIALOGEX 0, 0, 135, 62
+STYLE DS_SHELLFONT | DS_MODALFRAME | WS_POPUP | WS_CAPTION | WS_SYSMENU | WS_VISIBLE
+CAPTION "About Dialog_InitVisible"
+FONT 8, "MS Shell Dlg"
+BEGIN
+    LTEXT           "Dialog_InitVisible, Version 1.0", IDC_STATIC, 7, 14, 114, 8, SS_NOPREFIX
+    LTEXT           "Copyright (C) 2026 ReactOS", IDC_STATIC, 7, 26, 114, 8
+    DEFPUSHBUTTON   "OK", IDOK, 78, 41, 50, 14, WS_GROUP
+END

modules/rostests/win32/user32/dialog_initvisible/resource.h

@@ -0,0 +1,2 @@
+#define IDD_ABOUTBOX    101
+#define IDC_STATIC      -1