feat: Add UT for DComboBox

  Add UT.

tests/CMakeLists.txt

@@ -109,6 +109,7 @@ set(WIDGET_TEST
     testcases/widgets/ut_dwindowminbutton.cpp
     testcases/widgets/ut_dwindowoptionbutton.cpp
     testcases/widgets/ut_dwindowquitfullbutton.cpp
+    testcases/widgets/ut_dcombobox.cpp
 )
 
 include(${PROJECT_SOURCE_DIR}/src/util/util.cmake)

tests/testcases/widgets/ut_dcombobox.cpp

@@ -0,0 +1,48 @@
+// SPDX-FileCopyrightText: 2023 UnionTech Software Technology Co., Ltd.
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+
+#include <gtest/gtest.h>
+#include <QTest>
+
+#include "private/qcombobox_p.h"
+#include "dcombobox.h"
+DWIDGET_USE_NAMESPACE
+class ut_DComboBox : public testing::Test
+{
+protected:
+    void SetUp() override
+    {
+        target = new DComboBox();
+    }
+    void TearDown() override
+    {
+        if (target)
+            delete target;
+    }
+    DComboBox *target = nullptr;
+};
+
+TEST_F(ut_DComboBox, maxVisibleItems)
+{
+    // Add an additional item to display the arrow.
+    for (int i = 0; i < target->maxVisibleItems() + 1; i++) {
+        target->addItem(QString::number(i));
+    }
+    target->show();
+    QTest::qWaitForWindowExposed(target->windowHandle());
+
+    const QPoint arrowPos(target->rect().right() - 1, target->rect().center().y());
+    QTest::mouseClick(target, Qt::LeftButton, Qt::KeyboardModifiers(), arrowPos);
+
+    auto container = target->findChild<QComboBoxPrivateContainer *>();
+    ASSERT_TRUE(container);
+    const auto oldRect = container->rect();
+    target->hide();
+
+    target->addItem(QString("more"));
+    target->show();
+
+    QTest::mouseClick(target, Qt::LeftButton, Qt::KeyboardModifiers(), arrowPos);
+    ASSERT_EQ(container->rect().height(), oldRect.height());
+}