core/screen: retain removed screens to avoid use-after-free

src/core/qmlglobal.cpp

@@ -127,8 +127,10 @@ void QuickshellTracked::updateScreens() {
 	next:;
 	}
 
+	// Retain removed screens rather than deleting them, since users might hold on to
+	// a bare `QObject*` (via a `QVariant`) pointing at the old screen.
 	for (auto* oldScreen: this->screens) {
-		oldScreen->deleteLater();
+		this->mDeadScreens.push_back(oldScreen);
 	}
 
 	this->screens = newScreens;

src/core/qmlglobal.hpp

@@ -77,6 +77,11 @@ private slots:
 
 signals:
 	void screensChanged();
+
+private:
+	// Screens removed from the system are stored here instead of being
+	// deleted, since users might hold on to bare `QObject*`s pointing at them.
+	QVector<QuickshellScreenInfo*> mDeadScreens;
 };
 
 class QuickshellGlobal: public QObject {