feat: add option to provide a titlebar (default on)

app/titlebar.h

@@ -0,0 +1,60 @@
+// SPDX-FileCopyrightText: 2025 Gary Wang <git@blumia.net>
+//
+// SPDX-License-Identifier: MIT
+
+#ifndef TITLEBAR_H
+#define TITLEBAR_H
+
+#include <QWidget>
+#include <QIcon>
+
+QT_BEGIN_NAMESPACE
+class QPaintEvent;
+class QMouseEvent;
+class QEvent;
+class QEnterEvent;
+QT_END_NAMESPACE
+
+class OpacityHelper;
+
+class TitleBar : public QWidget
+{
+    Q_OBJECT
+public:
+    explicit TitleBar(QWidget *parent = nullptr);
+
+    void setOpacity(qreal opacity, bool animated = true);
+    void setCloseButtonVisible(bool visible);
+    bool closeButtonOnly() const { return m_closeButtonOnly; }
+    void setCloseButtonOnly(bool only);
+    int closeButtonWidth() const { return qMax(height(), 46); }
+
+signals:
+    void closeRequested();
+    void maximizeToggleRequested();
+
+protected:
+    void paintEvent(QPaintEvent *event) override;
+    void mousePressEvent(QMouseEvent *event) override;
+    void mouseMoveEvent(QMouseEvent *event) override;
+    void mouseReleaseEvent(QMouseEvent *event) override;
+    void mouseDoubleClickEvent(QMouseEvent *event) override;
+    void enterEvent(QEnterEvent *event) override;
+    void leaveEvent(QEvent *event) override;
+    bool eventFilter(QObject *watched, QEvent *event) override;
+    QSize sizeHint() const override;
+
+private:
+    QRect closeButtonRect() const;
+
+    OpacityHelper *m_opacityHelper;
+    QIcon m_closeIcon;
+    bool m_closeButtonVisible = true;
+    bool m_closeButtonOnly = false;
+    bool m_closeHovered = false;
+    bool m_closePressed = false;
+    bool m_dragPending = false;
+    QPoint m_moveStartPos;
+};
+
+#endif // TITLEBAR_H