fix(notifications): keep toast surface geometry stable

quickshell/.config/quickshell/labfy-sway/notifications/NotificationToastArea.qml

@@ -15,24 +15,40 @@ PopupWindow {
     anchor.edges: Edges.Top | Edges.Left
     anchor.gravity: Edges.Bottom | Edges.Right
     implicitWidth: 386
-    implicitHeight: Math.min(550, stack.implicitHeight + 16)
+    // WHY: redimensionner un popup Wayland déjà visible peut présenter un
+    // ancien buffer étiré pendant que Qt recalcule les cartes. La surface
+    // garde sa taille ; seule la pile interne suit les notifications.
+    // INVARIANT: hauteur native indépendante des arrivées et expirations.
+    implicitHeight: 550
     visible: service.toasts.length > 0
     grabFocus: false
     color: "transparent"
-    Column {
-        id: stack
+    // CONTRACT: la partie transparente ne capte pas les clics du bureau.
+    mask: Region { item: viewport }
+    Flickable {
+        id: viewport
         anchors.left: parent.left
         anchors.right: parent.right
         anchors.top: parent.top
         anchors.margins: 8
-        spacing: 7
-        Repeater {
-            model: area.service.toasts.slice(-4).reverse()
-            delegate: NotificationToast {
-                required property var modelData
-                entry: modelData
-                service: area.service
-                width: stack.width
+        height: Math.min(area.height - 16, contentHeight)
+        contentWidth: width
+        contentHeight: stack.childrenRect.height
+        clip: true
+        interactive: contentHeight > height
+        boundsBehavior: Flickable.StopAtBounds
+        Column {
+            id: stack
+            width: viewport.width
+            spacing: 7
+            Repeater {
+                model: area.service.toasts.slice(-4).reverse()
+                delegate: NotificationToast {
+                    required property var modelData
+                    entry: modelData
+                    service: area.service
+                    width: stack.width
+                }
             }
         }
     }