add basic UI indication of task scrub mode being active

includes a UI hint of what to do

kwin/mobiletaskswitcher/qml/TaskSwitcher.qml

@@ -430,6 +430,49 @@ FocusScope {
             }
         }
 
+        RowLayout {
+            id: scrubIndicator
+            opacity: taskSwitcherHelpers.isInTaskScrubMode ? 1 : 0
+            Behavior on opacity { NumberAnimation { duration: 200} }
+
+            anchors.bottom: parent.bottom
+            anchors.horizontalCenter: parent.horizontalCenter
+            anchors.bottomMargin: taskSwitcherHelpers.openedYPosition * 3 / 4
+
+            Kirigami.Icon {
+                id: iconScrubBack
+                opacity: root.state.currentTaskIndex == 0 ? 0.3 : 1
+                Behavior on opacity { NumberAnimation { duration: 200} }
+                Layout.alignment: Qt.AlignHCenter
+                implicitWidth: Kirigami.Units.iconSizes.small
+                implicitHeight: Kirigami.Units.iconSizes.small
+                source: "draw-arrow-back"
+                color: "white"
+            }
+
+            Kirigami.Heading {
+                Layout.fillWidth: true
+                Layout.maximumWidth: root.width * 0.75
+                Layout.alignment: Qt.AlignHCenter
+                color: "white"
+                level: 3
+                wrapMode: Text.Wrap
+                horizontalAlignment: Text.AlignHCenter
+                text: i18n("Swipe")
+            }
+
+            Kirigami.Icon {
+                id: iconScrubFront
+                opacity: root.state.currentTaskIndex == tasksCount - 1 ? 0.3 : 1
+                Behavior on opacity { NumberAnimation { duration: 200} }
+                Layout.alignment: Qt.AlignHCenter
+                implicitWidth: Kirigami.Units.iconSizes.small
+                implicitHeight: Kirigami.Units.iconSizes.small
+                source: "draw-arrow-forward"
+                color: "white"
+            }
+        }
+
         // flicking area for task switcher
         FlickContainer {
             id: flickable

kwin/mobiletaskswitcher/qml/TaskSwitcherHelpers.qml

@@ -31,7 +31,7 @@ QtObject {
     readonly property real openedYPosition: (taskSwitcher.height - taskHeight) / 2
 
     // yPosition threshold below which opening the task switcher should be undone and returned to the previously active task
-    readonly property real undoYThreshold: openedYPosition / 3
+    readonly property real undoYThreshold: openedYPosition / 2
 
     // ~~ measurement constants ~~