Tumbler: fix displacement calculation when wrap is false

Use the position of the item and the currentItem in the calculation
in order to get reliable results. This fixes the displacement being
off by a small margin, which increased as the delegate height became
smaller.

Fixes: QTBUG-66799
Change-Id: Ieca5033fb4c0ed62f5965a21fcab7aa558bd40e6
Reviewed-by: Andy Shaw <andy.shaw@qt.io>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

2 files changed, 38 insertions, 6 deletions

diff --git a/src/quicktemplates2/qquicktumbler.cpp b/src/quicktemplates2/qquicktumbler.cpp
index 8b702c60..85c70b1b 100644
--- a/src/quicktemplates2/qquicktumbler.cpp
+++ b/src/quicktemplates2/qquicktumbler.cpp
@@ -949,9 +949,20 @@ void QQuickTumblerAttachedPrivate::calculateDisplacement()
         const qreal contentY = tumblerPrivate->viewContentY;
         const qreal delegateH = delegateHeight(tumbler);
         const qreal preferredHighlightBegin = tumblerPrivate->view->property("preferredHighlightBegin").toReal();
-        // Tumbler's displacement goes from negative at the top to positive towards the bottom, so we must switch this around.
-        const qreal reverseDisplacement = (contentY + preferredHighlightBegin) / delegateH;
-        displacement = reverseDisplacement - index;
+        const qreal itemY = qobject_cast<QQuickItem*>(parent)->y();
+        qreal currentItemY = 0;
+        auto currentItem = tumblerPrivate->view->property("currentItem").value<QQuickItem*>();
+        if (currentItem)
+            currentItemY = currentItem->y();
+        // Start from the y position of the current item.
+        const qreal topOfCurrentItemInViewport = currentItemY - contentY;
+        // Then, calculate the distance between it and the preferredHighlightBegin.
+        const qreal relativePositionToPreferredHighlightBegin = topOfCurrentItemInViewport - preferredHighlightBegin;
+        // Next, calculate the distance between us and the current item.
+        const qreal distanceFromCurrentItem = currentItemY - itemY;
+        const qreal displacementInPixels = distanceFromCurrentItem - relativePositionToPreferredHighlightBegin;
+        // Convert it from pixels to a floating point index.
+        displacement = displacementInPixels / delegateH;
     }
 
     emitIfDisplacementChanged(previousDisplacement, displacement);
diff --git a/tests/auto/controls/data/tst_tumbler.qml b/tests/auto/controls/data/tst_tumbler.qml
index c9cc10d7..5b3ef6e3 100644
--- a/tests/auto/controls/data/tst_tumbler.qml
+++ b/tests/auto/controls/data/tst_tumbler.qml
@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2017 The Qt Company Ltd.
+** Copyright (C) 2019 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the test suite of the Qt Toolkit.
@@ -54,8 +54,8 @@ import QtQuick.Controls 2.12
 
 TestCase {
     id: testCase
-    width: 200
-    height: 200
+    width: 300
+    height: 300
     visible: true
     when: windowShown
     name: "Tumbler"
@@ -513,6 +513,7 @@ TestCase {
         Text {
             text: parent.displacement.toFixed(2)
             anchors.right: parent.right
+            anchors.verticalCenter: parent.verticalCenter
         }
 
         property real displacement: Tumbler.displacement
@@ -1236,4 +1237,24 @@ TestCase {
         // 5 - 2 = 3
         compare(tumbler.currentIndex, 3);
     }
+
+    function test_displacementAfterResizing() {
+        createTumbler({
+            width: 200,
+            wrap: false,
+            delegate: displacementDelegate,
+            model: 30,
+            visibleItemCount: 7,
+            currentIndex: 15
+        })
+
+        var delegate = findChild(tumblerView, "delegate15")
+        verify(delegate)
+
+        tryCompare(delegate, "displacement", 0)
+
+        // Resizing the Tumbler shouldn't affect the displacement.
+        tumbler.height *= 1.4
+        tryCompare(delegate, "displacement", 0)
+    }
 }