Fade the dock thumbnail popup in and out

The popup snapped to visible/hidden instantly while every other
surface in the shell uses animated transitions. Add an opacity
fade over shortDuration so it matches the rest of the motion
language. State cleanup waits for the fade-out to finish.

containments/homescreens/folio/qml/FavouritesBar.qml

@@ -599,7 +599,7 @@ MouseArea {
         id: thumbnailShowTimer
         interval: Kirigami.Units.toolTipDelay
         onTriggered: {
-            thumbnailPopup.visible = true
+            thumbnailPopup.showing = true
         }
     }
 
@@ -607,7 +607,7 @@ MouseArea {
         id: thumbnailHideTimer
         interval: 300
         onTriggered: {
-            thumbnailPopup.visible = false
+            thumbnailPopup.showing = false
             root.hoveredTaskIndex = -1
         }
     }
@@ -620,10 +620,21 @@ MouseArea {
         property var windowIds: []
         property bool isGroup: false
         property bool popupHovered: false
+        property bool showing: false
 
-        function open() { visible = true }
-        function close() { visible = false }
-        readonly property bool opened: visible
+        function open() { showing = true }
+        function close() { showing = false }
+        readonly property bool opened: showing
+
+        visible: showing || fadeAnim.running
+        opacity: showing ? 1 : 0
+        Behavior on opacity {
+            NumberAnimation {
+                id: fadeAnim
+                duration: Kirigami.Units.shortDuration
+                easing.type: Easing.InOutQuad
+            }
+        }
 
         flags: Qt.ToolTip | Qt.FramelessWindowHint | Qt.WindowDoesNotAcceptFocus
         color: "transparent"
@@ -650,8 +661,8 @@ MouseArea {
             return Math.max(0, Math.min(Screen.height - height, delegateGlobal.y - height - Kirigami.Units.smallSpacing))
         }
 
-        onVisibleChanged: {
-            if (!visible) {
+        onShowingChanged: {
+            if (!showing && !fadeAnim.running) {
                 windowIds = []
                 targetDelegate = null
                 taskIndex = -1
@@ -659,6 +670,18 @@ MouseArea {
             }
         }
 
+        Connections {
+            target: fadeAnim
+            function onRunningChanged() {
+                if (!fadeAnim.running && !thumbnailPopup.showing) {
+                    thumbnailPopup.windowIds = []
+                    thumbnailPopup.targetDelegate = null
+                    thumbnailPopup.taskIndex = -1
+                    thumbnailPopup.isGroup = false
+                }
+            }
+        }
+
         Rectangle {
             anchors.fill: parent
             color: Kirigami.Theme.backgroundColor