Take overshoot into account when laying out QAbstractScrollArea

QAbstractScrollAreaPrivate::layoutChildren() positions the viewport, but
did not take the overshoot from scrolling with a scroller into account.
If the scroll area was resized during a scroll, then this resulted in the
roll back overcompensating for the overshoot, placing the viewport outside
the visible area.

Fix this by taking the overshoot into account when positioning the
viewport.

Add a test case. We have to use QWindow-based mouse event simulation, as
the QWidget based move events use QCursor::setPos, which doesn't reliably
go through the gesture framework.

Done-with: Volker Hilsheimer <volker.hilsheimer@qt.io>
Done-with: Zhang Hao <zhanghao@uniontech.com>
Fixes: QTBUG-94769
Pick-to: 5.15 6.2
Change-Id: Idf650c91e5a9cffa996e23e743939243b1d4fcc0
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 39 insertions, 0 deletions

diff --git a/tests/auto/widgets/widgets/qabstractscrollarea/tst_qabstractscrollarea.cpp b/tests/auto/widgets/widgets/qabstractscrollarea/tst_qabstractscrollarea.cpp
index de57769793..9d7cc484cd 100644
--- a/tests/auto/widgets/widgets/qabstractscrollarea/tst_qabstractscrollarea.cpp
+++ b/tests/auto/widgets/widgets/qabstractscrollarea/tst_qabstractscrollarea.cpp
@@ -37,6 +37,7 @@
 #include <qlabel.h>
 #include <qwidget.h>
 #include <qdialog.h>
+#include <qscroller.h>
 
 class tst_QAbstractScrollArea : public QObject
 {
@@ -57,6 +58,7 @@ private slots:
     void task214488_layoutDirection();
 
     void margins();
+    void resizeWithOvershoot();
 };
 
 tst_QAbstractScrollArea::tst_QAbstractScrollArea()
@@ -394,5 +396,42 @@ void tst_QAbstractScrollArea::margins()
     QCOMPARE(area.viewportMargins(), margins);
 }
 
+void tst_QAbstractScrollArea::resizeWithOvershoot()
+{
+    QWidget window;
+
+    QScrollArea scrollArea(&window);
+    scrollArea.setWidget([]{
+        QWidget *widget = new QWidget;
+        widget->setFixedSize(QSize(0, 200));
+        return widget;
+    }());
+    scrollArea.setGeometry(0, 20, 100, 100);
+
+    QScroller::grabGesture(&scrollArea, QScroller::LeftMouseButtonGesture);
+
+    window.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&window));
+
+    const QPoint originAtRest = scrollArea.viewport()->pos();
+
+    QPoint center = scrollArea.viewport()->mapToGlobal(scrollArea.viewport()->rect().center());
+    center = window.windowHandle()->mapFromGlobal(center);
+    QTest::mousePress(window.windowHandle(), Qt::LeftButton, {}, center);
+    QTest::mouseMove(window.windowHandle(), center + QPoint(0, 50));
+    QTRY_COMPARE(scrollArea.viewport()->pos(), originAtRest + QPoint(0, 25));
+    QPoint overshootPosition = scrollArea.viewport()->pos();
+
+    // trigger a layout of the scroll area while there's overshoot
+    scrollArea.setGeometry(0, 0, 100, 120);
+    QCOMPARE(scrollArea.viewport()->pos(), overshootPosition);
+    QTest::mouseRelease(window.windowHandle(), Qt::LeftButton, {}, center + QPoint(0, 50));
+    QTRY_COMPARE(scrollArea.viewport()->pos(), originAtRest);
+    // Process a few more events and verify that the scroll area
+    // doesn't overcompensate for the overshoot.
+    QApplication::processEvents();
+    QTRY_COMPARE(scrollArea.viewport()->pos(), originAtRest);
+}
+
 QTEST_MAIN(tst_QAbstractScrollArea)
 #include "tst_qabstractscrollarea.moc"