Give notification popups a fixed-size surface to stop resize scaling

The popups used one Wayland layer surface per output sized to their content
(implicitWidth/Height bound to the toast column), so every time a toast was
added or removed the surface resized. During that resize the compositor
briefly scaled a stale buffer, stretching or squishing the cards mid-
transition -- worst with only one or two toasts, where each resize is a
larger relative change.

Make the surface full-screen and fixed, like the OSD overlay, so adding or
removing a toast only changes the content inside it and the surface never
resizes. Mask the input region to the toast column so the otherwise-
invisible full-screen overlay stays click-through.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

shell/plugins/notifications/Service.qml

@@ -787,26 +787,27 @@ Item {
       readonly property var popupPlacement: NotificationLogic.popupPlacement(
         service.barPosition, service.barClearance, Style.gapsOut)
 
-      anchors {
+      // Full-screen, fixed-size surface (like the OSD overlay). Adding or
-        top: popupWindow.popupPlacement.anchors.top
+      // removing a toast changes only the content inside; the Wayland surface
-        bottom: popupWindow.popupPlacement.anchors.bottom
+      // never resizes, so the compositor can't briefly scale a stale buffer --
-        left: popupWindow.popupPlacement.anchors.left
+      // which is what stretched/squished the cards during count changes.
-        right: popupWindow.popupPlacement.anchors.right
+      anchors { top: true; bottom: true; left: true; right: true }
-      }
-      margins {
-        top: popupWindow.popupPlacement.margins.top
-        bottom: popupWindow.popupPlacement.margins.bottom
-        left: popupWindow.popupPlacement.margins.left
-        right: popupWindow.popupPlacement.margins.right
-      }
 
-      implicitWidth: popupColumn.implicitWidth
+      // Keep the surface click-through except over the toast column, so the
-      implicitHeight: popupColumn.implicitHeight
+      // rest of the (invisible) full-screen overlay never eats input.
+      mask: Region {
+        x: popupColumn.x
+        y: popupColumn.y
+        width: popupColumn.width
+        height: popupColumn.height
+      }
 
       ColumnLayout {
         id: popupColumn
         anchors.right: parent.right
         anchors.top: parent.top
+        anchors.topMargin: popupWindow.popupPlacement.margins.top
+        anchors.rightMargin: popupWindow.popupPlacement.margins.right
         spacing: Style.space(8)
 
         Repeater {