taskmanager: Update rootIndex when visibility is changed

When there are two grouped tasks named A and B, and the position
of B is swapped with A (by dragging B), user can still see old
items of A in the textual list of grouped tasks B.

This is because in the original binding expression, both visualParent
and itemIndex are not changed, so QML will not re-evaluate the
expression.

To only update rootIndex when `groupDialog.visible` is true,
`Binding.when` is used, and `Binding.RestoreNone` is to avoid
sudden changes in height.

BUG: 446108

applets/taskmanager/package/contents/ui/GroupDialog.qml

@@ -6,6 +6,7 @@
 */
 
 import QtQuick 2.15
+import QtQml 2.15
 import QtQml.Models 2.2
 import QtQuick.Window 2.2
 
@@ -82,7 +83,11 @@ PlasmaCore.Dialog {
                     property int maxTextWidth: 0
 
                     model: groupDialog.visualParent ? tasksModel : null
-                    rootIndex: groupDialog.visualParent ? tasksModel.makeModelIndex(groupDialog.visualParent.itemIndex) : null
+                    Binding on rootIndex { // Use Binding to re-evaluate rootIndex only when visible is true
+                        value: tasksModel.makeModelIndex(groupDialog.visualParent.itemIndex)
+                        when: groupDialog.visualParent && groupDialog.visible // implicitly using visible as a way to trigger re-evaluation
+                        restoreMode: Binding.RestoreNone // Store rootIndex to avoid sudden changes in height
+                    }
                     delegate: Task {
                         visible: true
                         inPopup: true

[ivan tkachenko]

@fusionfuture Can I kindly ask you to add a boilerplate comment when doing this QtQml magic?

// Deliberately imported after QtQuick to avoid missing restoreMode property in Binding. Fix in Qt 6.

[Fushan Wen]

Eh, it seems too long. What about // TODO: Remove in Qt 6?

[ivan tkachenko]

This is what Qt uses upstream, so it makes sense to just stick with it. I'm pushing it to master, alight?

[Fushan Wen]

Fine

[ivan tkachenko]

Done: 9cfd6141