add haptic feedback to task switcher gestures

haptics are triggered when conditions are met to open task switcher or task scrub mode is engaged.
fixes related bug in task switcher gesture logic when invoked from homescreen

fixes https://invent.kde.org/plasma/plasma-mobile/-/issues/366

kwin/mobiletaskswitcher/qml/TaskSwitcher.qml

@@ -39,6 +39,10 @@ FocusScope {
         stateClass: TaskSwitcherData.TaskSwitcherState
     }
 
+    MobileShell.HapticsEffect {
+        id: haptics
+    }
+
     property var tasksModel: TaskSwitcherData.TaskFilterModel {
         screenName: root.targetScreen.name
         windowModel: TaskSwitcherData.TaskModel
@@ -102,6 +106,12 @@ FocusScope {
                 taskSwitcherHelpers.isInTaskScrubMode = true;
                 taskSwitcherHelpers.cancelAnimations();
                 taskSwitcherHelpers.open();
+                if (!taskSwitcherHelpers.hasVibrated) {
+                    // Haptic feedback when the task scrub mode engages
+                    haptics.buttonVibrate();
+                    taskSwitcherHelpers.hasVibrated = true;
+                }
+
             }
             let newTaskIndex = Math.max(0, Math.min(tasksCount - 1, Math.floor(state.touchXPosition / taskSwitcherHelpers.taskScrubDistance) + state.initialTaskIndex));
             if (newTaskIndex != state.currentTaskIndex) {
@@ -217,12 +227,33 @@ FocusScope {
                         taskSwitcherHelpers.open();
                     } else {
                         // no flick and not enough activation to go to task switcher
-                        returnToApp();
+                        if (state.wasInActiveTask) {
+                            returnToApp();
+                        } else {
+                            // do open switcher in case we were on homescreen before
+                            taskSwitcherHelpers.animateGoToTaskIndex(state.currentTaskIndex);
+                            taskSwitcherHelpers.open();
+                        }
+
                     }
                 }
             }
         }
 
+        function onVelocityChanged() {
+            if (!taskSwitcherHelpers.hasVibrated) {
+                if (!state.wasInActiveTask ||
+                    (state.wasInActiveTask &&
+                     state.yPosition > taskSwitcherHelpers.undoYThreshold &&
+                     state.totalSquaredVelocity < state.flickVelocityThreshold)) {
+                        // Haptic feedback when conditions are met for the task switcher to open
+                        haptics.buttonVibrate();
+                        taskSwitcherHelpers.hasVibrated = true;
+                }
+
+            }
+        }
+
         function onXPositionChanged() {
             taskSwitcherHelpers.updateTaskIndex();
         }

kwin/mobiletaskswitcher/qml/TaskSwitcherHelpers.qml

@@ -33,6 +33,10 @@ QtObject {
     // yPosition threshold below which opening the task switcher should be undone and returned to the previously active task
     readonly property real undoYThreshold: openedYPosition / 2
 
+    // whether the switcher has already triggered haptic feedback or not
+    // we don't want to continuously send haptics, just once is enough
+    property bool hasVibrated: false
+
     // ~~ measurement constants ~~
 
     // dimensions of a real window on the screen