QAbstractItemView: with single selection, deselect on Ctrl+Release

After cbf1b4bc60bca3994b8f8685ee922e53a6b4eed2 the selected item got
deselected on Ctrl+Press, which made Ctrl+dragging a selected item
impossible.

Only deselect on Ctrl+Release. Add scenario to existing test case,
and update the documentation to clarify the properties involved, and
to point out that the event parameter might be nullptr.

Fixes: QTBUG-101647
Pick-to: 6.3 6.2
Change-Id: I749b1cb1a0a311f5c1d4c333984716f05f2c90b5
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Volker Enderlein <volker.enderlein@ifm-chemnitz.de>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

2 files changed, 30 insertions, 9 deletions

diff --git a/src/widgets/itemviews/qabstractitemview.cpp b/src/widgets/itemviews/qabstractitemview.cpp
index 16b5c26096..a4dee5e48c 100644
--- a/src/widgets/itemviews/qabstractitemview.cpp
+++ b/src/widgets/itemviews/qabstractitemview.cpp
@@ -4086,9 +4086,10 @@ void QAbstractItemView::doAutoScroll()
 }
 
 /*!
-    Returns the SelectionFlags to be used when updating a selection with
-    to include the \a index specified. The \a event is a user input event,
-    such as a mouse or keyboard event.
+    Returns the SelectionFlags to be used when updating a selection model
+    for the specified \a index. The result depends on the current
+    selectionMode(), and on the user input event \a event, which can be
+    \nullptr.
 
     Reimplement this function to define your own selection behavior.
 
@@ -4105,12 +4106,24 @@ QItemSelectionModel::SelectionFlags QAbstractItemView::selectionCommand(const QM
         case NoSelection: // Never update selection model
             return QItemSelectionModel::NoUpdate;
         case SingleSelection: // ClearAndSelect on valid index otherwise NoUpdate
-            if (event && event->type() == QEvent::MouseButtonRelease)
-                return QItemSelectionModel::NoUpdate;
-            if ((keyModifiers & Qt::ControlModifier) && d->selectionModel->isSelected(index) && event->type() != QEvent::MouseMove)
-                return QItemSelectionModel::Deselect | d->selectionBehaviorFlags();
-            else
-                return QItemSelectionModel::ClearAndSelect | d->selectionBehaviorFlags();
+            if (event) {
+                switch (event->type()) {
+                case QEvent::MouseButtonPress:
+                    // press with any modifiers on a selected item does nothing
+                    if (d->pressedAlreadySelected)
+                        return QItemSelectionModel::NoUpdate;
+                    break;
+                case QEvent::KeyPress:
+                case QEvent::MouseButtonRelease:
+                    // ctrl-release on selected item deselects
+                    if ((keyModifiers & Qt::ControlModifier) && d->selectionModel->isSelected(index))
+                        return QItemSelectionModel::Deselect | d->selectionBehaviorFlags();
+                    break;
+                default:
+                    break;
+                }
+            }
+            return QItemSelectionModel::ClearAndSelect | d->selectionBehaviorFlags();
         case MultiSelection:
             return d->multiSelectionCommand(index, event);
         case ExtendedSelection:
diff --git a/tests/auto/widgets/itemviews/qabstractitemview/tst_qabstractitemview.cpp b/tests/auto/widgets/itemviews/qabstractitemview/tst_qabstractitemview.cpp
index 869d552a4f..dbc27368f4 100644
--- a/tests/auto/widgets/itemviews/qabstractitemview/tst_qabstractitemview.cpp
+++ b/tests/auto/widgets/itemviews/qabstractitemview/tst_qabstractitemview.cpp
@@ -2889,9 +2889,17 @@ void tst_QAbstractItemView::mouseSelection_data()
     QTest::addRow("Single:Shift+Click") << QAbstractItemView::SingleSelection << false
         << QList{SelectionEvent{SelectionEvent::Click, Qt::ShiftModifier, 2}}
         << QList{2};
+    QTest::addRow("Single:Press;Ctrl+Press") << QAbstractItemView::SingleSelection << false
+        << QList{SelectionEvent{SelectionEvent::Press, 3},
+                 SelectionEvent{SelectionEvent::Press, Qt::ControlModifier, 3}}
+        << QList{3};
     QTest::addRow("Single:Ctrl+Click") << QAbstractItemView::SingleSelection << false
         << QList{SelectionEvent{SelectionEvent::Click, Qt::ControlModifier, 3}}
         << QList{3};
+    QTest::addRow("Single:Click;Ctrl+Click") << QAbstractItemView::SingleSelection << false
+        << QList{SelectionEvent{SelectionEvent::Click, 3},
+                 SelectionEvent{SelectionEvent::Click, Qt::ControlModifier, 3}}
+        << QList<int>{};
 
     // multi selection mode - selection toggles on press, selection can be drag-extended
     // modifiers ignored