core/window: add parentWindow property to FloatingWindow

src/window/floatingwindow.cpp

@@ -2,7 +2,7 @@
 
 #include <qobject.h>
 #include <qqmlengine.h>
-#include <qqmllist.h>
+#include <qqmlinfo.h>
 #include <qtmetamacros.h>
 #include <qtypes.h>
 
@@ -17,6 +17,13 @@ void ProxyFloatingWindow::connectWindow() {
 	this->window->setMaximumSize(this->bMaximumSize);
 }
 
+void ProxyFloatingWindow::postCompleteWindow() {
+	auto* parentBacking =
+	    this->mParentProxyWindow ? this->mParentProxyWindow->backingWindow() : nullptr;
+
+	this->window->setTransientParent(parentBacking);
+}
+
 void ProxyFloatingWindow::trySetWidth(qint32 implicitWidth) {
 	if (!this->window->isVisible()) {
 		this->ProxyWindowBase::trySetWidth(implicitWidth);
@@ -44,6 +51,28 @@ void ProxyFloatingWindow::onMaximumSizeChanged() {
 	emit this->maximumSizeChanged();
 }
 
+QObject* ProxyFloatingWindow::parentWindow() const { return this->mParentWindow; }
+
+void ProxyFloatingWindow::setParentWindow(QObject* window) {
+	if (window == this->mParentWindow) return;
+
+	if (window) {
+		if (auto* proxy = qobject_cast<ProxyWindowBase*>(window)) {
+			this->mParentProxyWindow = proxy;
+		} else if (auto* interface = qobject_cast<WindowInterface*>(window)) {
+			this->mParentProxyWindow = interface->proxyWindow();
+		} else {
+			qmlWarning(this) << "parentWindow must be a quickshell window.";
+			return;
+		}
+
+		this->mParentWindow = window;
+	} else {
+		this->mParentWindow = nullptr;
+		this->mParentProxyWindow = nullptr;
+	}
+}
+
 // FloatingWindowInterface
 
 FloatingWindowInterface::FloatingWindowInterface(QObject* parent)
@@ -66,3 +95,9 @@ void FloatingWindowInterface::onReload(QObject* oldInstance) {
 }
 
 ProxyWindowBase* FloatingWindowInterface::proxyWindow() const { return this->window; }
+
+QObject* FloatingWindowInterface::parentWindow() const { return this->window->parentWindow(); }
+
+void FloatingWindowInterface::setParentWindow(QObject* window) {
+	this->window->setParentWindow(window);
+}

src/window/floatingwindow.hpp

@@ -18,6 +18,10 @@ class ProxyFloatingWindow: public ProxyWindowBase {
 	explicit ProxyFloatingWindow(QObject* parent = nullptr): ProxyWindowBase(parent) {}
 
 	void connectWindow() override;
+	void postCompleteWindow() override;
+
+	[[nodiscard]] QObject* parentWindow() const;
+	void setParentWindow(QObject* window);
 
 	// Setting geometry while the window is visible makes the content item shrink but not the window
 	// which is awful so we disable it for floating windows.
@@ -34,6 +38,9 @@ class ProxyFloatingWindow: public ProxyWindowBase {
 	void onMaximumSizeChanged();
 	void onTitleChanged();
 
+	QObject* mParentWindow = nullptr;
+	ProxyWindowBase* mParentProxyWindow = nullptr;
+
 public:
 	Q_OBJECT_BINDABLE_PROPERTY(
 	    ProxyFloatingWindow,
@@ -68,6 +75,11 @@ class FloatingWindowInterface: public WindowInterface {
 	Q_PROPERTY(QSize minimumSize READ default WRITE default NOTIFY minimumSizeChanged BINDABLE bindableMinimumSize);
 	/// Maximum window size given to the window system.
 	Q_PROPERTY(QSize maximumSize READ default WRITE default NOTIFY maximumSizeChanged BINDABLE bindableMaximumSize);
+	/// The parent window of this window. Setting this makes the window a child of the parent,
+	/// which affects window stacking behavior.
+	///
+	/// > [!NOTE] This property cannot be changed after the window is visible.
+	Q_PROPERTY(QObject* parentWindow READ parentWindow WRITE setParentWindow);
 	// clang-format on
 	QML_NAMED_ELEMENT(FloatingWindow);
 
@@ -82,6 +94,9 @@ class FloatingWindowInterface: public WindowInterface {
 	QBindable<QSize> bindableMaximumSize() { return &this->window->bMaximumSize; }
 	QBindable<QString> bindableTitle() { return &this->window->bTitle; }
 
+	[[nodiscard]] QObject* parentWindow() const;
+	void setParentWindow(QObject* window);
+
 signals:
 	void minimumSizeChanged();
 	void maximumSizeChanged();