Fix scroll bars not showing up when binding to standalone contentItem

908aa77d16e00f2bccc0ddae0f8b61955c56a6a1 hid old scroll bars, but
didn't account for the situation where the old scroll bars would be put
back into place, and so they never showed up.

In the case of the linked bug report, since there was a binding to the
ScrollView's contentItem, a default Flickable would be created. After
that binding was evaluated, the contentItem was set, causing the scroll
bars to be hidden (as part of the process of disconnecting from the old
flickable). To fix the issue, we now do the reverse of hideOldItem when
a new contentItem is set.

Fixes: QTBUG-104983
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Nate Graham
(cherry picked from commit 58bae53237417f28eac6d772fa6ecab657f8a73f)
Change-Id: I0d5d04cf9268e03b99c8b8fba2eee407e225ae56

5 files changed, 91 insertions, 0 deletions

diff --git a/src/quicktemplates2/qquickcontrol.cpp b/src/quicktemplates2/qquickcontrol.cpp
index ac1303a2..23ebf345 100644
--- a/src/quicktemplates2/qquickcontrol.cpp
+++ b/src/quicktemplates2/qquickcontrol.cpp
@@ -853,6 +853,13 @@ void QQuickControlPrivate::executeBackground(bool complete)
         quickCompleteDeferred(q, backgroundName(), background);
 }
 
+/*
+    \internal
+
+    Hides an item that was replaced by a newer one, rather than
+    deleting it, as the item is typically created in QML and hence
+    we don't own it.
+*/
 void QQuickControlPrivate::hideOldItem(QQuickItem *item)
 {
     if (!item)
@@ -871,6 +878,29 @@ void QQuickControlPrivate::hideOldItem(QQuickItem *item)
 #endif
 }
 
+/*
+    \internal
+
+    Named "unhide" because it's used for cases where an item
+    that was previously hidden by \l hideOldItem() wants to be
+    shown by a control again, such as a ScrollBar in ScrollView.
+*/
+void QQuickControlPrivate::unhideOldItem(QQuickControl *control, QQuickItem *item)
+{
+    Q_ASSERT(item);
+    qCDebug(lcItemManagement) << "unhiding old item" << item;
+
+    item->setVisible(true);
+    item->setParentItem(control);
+
+#if QT_CONFIG(accessibility)
+    // Add the item back in to the accessibility tree.
+    QQuickAccessibleAttached *accessible = accessibleAttached(item);
+    if (accessible)
+        accessible->setIgnored(false);
+#endif
+}
+
 void QQuickControlPrivate::updateBaselineOffset()
 {
     Q_Q(QQuickControl);
diff --git a/src/quicktemplates2/qquickcontrol_p_p.h b/src/quicktemplates2/qquickcontrol_p_p.h
index aca6d03f..a923aa18 100644
--- a/src/quicktemplates2/qquickcontrol_p_p.h
+++ b/src/quicktemplates2/qquickcontrol_p_p.h
@@ -173,6 +173,7 @@ public:
     virtual void executeBackground(bool complete = false);
 
     static void hideOldItem(QQuickItem *item);
+    static void unhideOldItem(QQuickControl *control, QQuickItem *item);
 
     void updateBaselineOffset();
 
diff --git a/src/quicktemplates2/qquickscrollbar.cpp b/src/quicktemplates2/qquickscrollbar.cpp
index c9016cc9..ec647407 100644
--- a/src/quicktemplates2/qquickscrollbar.cpp
+++ b/src/quicktemplates2/qquickscrollbar.cpp
@@ -797,6 +797,14 @@ void QQuickScrollBarAttachedPrivate::initHorizontal()
     if (parent && parent == flickable->parentItem())
         horizontal->stackAfter(flickable);
 
+    // If a scroll bar was previously hidden (due to e.g. setting a new contentItem
+    // on a ScrollView), we need to make sure that we un-hide it.
+    // We don't bother checking if the item is actually the old one, because
+    // if it's not, all of the things the function does (setting parent, visibility, etc.)
+    // should be no-ops anyway.
+    if (auto control = qobject_cast<QQuickControl*>(q_func()->parent()))
+        QQuickControlPrivate::unhideOldItem(control, horizontal);
+
     layoutHorizontal();
     horizontal->setSize(area->property("widthRatio").toReal());
     horizontal->setPosition(area->property("xPosition").toReal());
@@ -818,6 +826,9 @@ void QQuickScrollBarAttachedPrivate::initVertical()
     if (parent && parent == flickable->parentItem())
         vertical->stackAfter(flickable);
 
+    if (auto control = qobject_cast<QQuickControl*>(q_func()->parent()))
+        QQuickControlPrivate::unhideOldItem(control, vertical);
+
     layoutVertical();
     vertical->setSize(area->property("heightRatio").toReal());
     vertical->setPosition(area->property("yPosition").toReal());
diff --git a/src/quicktemplates2/qquickscrollbar_p_p.h b/src/quicktemplates2/qquickscrollbar_p_p.h
index 6841ec73..bbb5f6c1 100644
--- a/src/quicktemplates2/qquickscrollbar_p_p.h
+++ b/src/quicktemplates2/qquickscrollbar_p_p.h
@@ -107,6 +107,8 @@ public:
 
 class QQuickScrollBarAttachedPrivate : public QObjectPrivate, public QQuickItemChangeListener
 {
+    Q_DECLARE_PUBLIC(QQuickScrollBar)
+
 public:
     static QQuickScrollBarAttachedPrivate *get(QQuickScrollBarAttached *attached)
     {
diff --git a/tests/auto/controls/data/tst_scrollview.qml b/tests/auto/controls/data/tst_scrollview.qml
index 0e8b0835..cd493118 100644
--- a/tests/auto/controls/data/tst_scrollview.qml
+++ b/tests/auto/controls/data/tst_scrollview.qml
@@ -576,4 +576,51 @@ TestCase {
         verify(newHorizontalScrollBar.visible)
         verify(!oldHorizontalScrollBar.visible)
     }
+
+    Component {
+        id: bindingToContentItemAndStandaloneFlickable
+
+        Item {
+            width: 200
+            height: 200
+
+            property alias scrollView: scrollView
+
+            ScrollView {
+                id: scrollView
+                anchors.fill: parent
+                contentItem: listView
+
+                property Item someBinding: contentItem
+            }
+            ListView {
+                id: listView
+                model: 10
+                delegate: ItemDelegate {
+                    text: modelData
+                    width: listView.width
+                }
+            }
+        }
+    }
+
+    // Tests that scroll bars show up for a ScrollView where
+    // - its contentItem is declared as a standalone, separate item
+    // - there is a binding to contentItem (which causes a default Flickable to be created)
+    function test_bindingToContentItemAndStandaloneFlickable() {
+        let root = createTemporaryObject(bindingToContentItemAndStandaloneFlickable, testCase)
+        verify(root)
+
+        let control = root.scrollView
+        let verticalScrollBar = control.ScrollBar.vertical
+        let horizontalScrollBar = control.ScrollBar.horizontal
+        compare(verticalScrollBar.parent, control)
+        compare(horizontalScrollBar.parent, control)
+        verify(verticalScrollBar.visible)
+        verify(horizontalScrollBar.visible)
+
+        mouseDrag(verticalScrollBar, verticalScrollBar.width / 2, verticalScrollBar.height / 2, 0, 50)
+        verify(verticalScrollBar.active)
+        verify(horizontalScrollBar.active)
+    }
 }