QTabBar: Improve scrolling with high resolution mouse wheels

The current behavior for handling the angle delta of a wheel event
changes index the instant there is a change in angle delta. This works
fine for mouse wheels that send events with 120 angle delta units and
there is also already behavior defined for devices with pixel deltas,
but there is nothing good for handling events from high resolution mouse
wheels that don't have pixel deltas.

This patch makes it so that the current index doesn't change until the
accumulated angle delta for the X or Y axis reaches 120.

[ChangeLog][QtWidgets][QTabBar] Scrolling with a high resolution mouse
wheel changes the current index at a rate more like a normal mouse
wheel.

Task-number: QTBUG-97844
Pick-to: 6.3
Change-Id: I2e7fd88984a253f6ef8a0008deb7233e4cb4d84a
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/widgets/qtabbar.cpp

@@ -1718,6 +1718,9 @@ bool QTabBar::event(QEvent *event)
             update(d->hoverRect);
         d->hoverIndex = -1;
         d->hoverRect = QRect();
+#if QT_CONFIG(wheelevent)
+        d->accumulatedAngleDelta = QPoint();
+#endif
         return true;
     }
 #if QT_CONFIG(tooltip)
@@ -2421,8 +2424,17 @@ void QTabBar::wheelEvent(QWheelEvent *event)
                 }
             }
         } else {
-            const int delta = wheelVertical ? event->angleDelta().y() : event->angleDelta().x();
+            d->accumulatedAngleDelta += event->angleDelta();
-            const int offset = delta > 0 ? -1 : 1;
+            const int xSteps = d->accumulatedAngleDelta.x() / QWheelEvent::DefaultDeltasPerStep;
+            const int ySteps = d->accumulatedAngleDelta.y() / QWheelEvent::DefaultDeltasPerStep;
+            int offset = 0;
+            if (xSteps > 0 || ySteps > 0) {
+                offset = -1;
+                d->accumulatedAngleDelta = QPoint();
+            } else if (xSteps < 0 || ySteps < 0) {
+                offset = 1;
+                d->accumulatedAngleDelta = QPoint();
+            }
             const int oldCurrentIndex = d->currentIndex;
             d->setCurrentNextEnabledIndex(offset);
             if (oldCurrentIndex != d->currentIndex) {

src/widgets/widgets/qtabbar_p.h

@@ -102,6 +102,9 @@ public:
     QRect hoverRect;
     QPoint dragStartPosition;
     QPoint mousePosition = {-1, -1};
+#if QT_CONFIG(wheelevent)
+    QPoint accumulatedAngleDelta;
+#endif
     QSize iconSize;
     QToolButton* rightB = nullptr; // right or bottom
     QToolButton* leftB = nullptr; // left or top

tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp

@@ -103,6 +103,8 @@ private slots:
     void mouseWheel();
     void kineticWheel_data();
     void kineticWheel();
+    void highResolutionWheel_data();
+    void highResolutionWheel();
 
     void scrollButtons_data();
     void scrollButtons();
@@ -1106,6 +1108,49 @@ void tst_QTabBar::kineticWheel()
     }
 }
 
+void tst_QTabBar::highResolutionWheel_data()
+{
+    QTest::addColumn<int>("angleDelta");
+    // Smallest angleDelta for a Logitech MX Master 3 with Linux/X11/Libinput
+    QTest::addRow("increment index") << -16;
+    QTest::addRow("decrement index") << 16;
+}
+
+void tst_QTabBar::highResolutionWheel()
+{
+    TabBar tabbar;
+    TabBarScrollingProxyStyle proxyStyle;
+    tabbar.setStyle(&proxyStyle);
+
+    tabbar.addTab("tab1");
+    tabbar.addTab("tab2");
+    QFETCH(int, angleDelta);
+    // Negative values increment, positive values decrement
+    int startIndex = angleDelta < 0 ? 0 : 1;
+    tabbar.setCurrentIndex(startIndex);
+
+    const auto systemId = QPointingDevice::primaryPointingDevice()->systemId() + 1;
+    QPointingDevice hiResWheel(
+            "test high resolution wheel", systemId, QInputDevice::DeviceType::Mouse,
+            QPointingDevice::PointerType::Generic,
+            QInputDevice::Capability::Position | QInputDevice::Capability::Scroll, 1, 3);
+
+    const QPoint wheelPoint = tabbar.rect().bottomRight();
+    QWheelEvent event(wheelPoint, tabbar.mapToGlobal(wheelPoint), QPoint(),
+                      QPoint(angleDelta, angleDelta), Qt::NoButton, Qt::NoModifier,
+                      Qt::NoScrollPhase, false, Qt::MouseEventSynthesizedByApplication,
+                      &hiResWheel);
+
+    proxyStyle.scrolling = true;
+    for (int accumulated = 0; accumulated < QWheelEvent::DefaultDeltasPerStep;
+         accumulated += qAbs(angleDelta)) {
+        // verify that nothing has changed until the threshold has been reached
+        QVERIFY(tabbar.currentIndex() == startIndex);
+        QVERIFY(QApplication::sendEvent(&tabbar, &event));
+    }
+    QVERIFY(tabbar.currentIndex() != startIndex);
+}
+
 #endif // QT_CONFIG(wheelevent)
 
 void tst_QTabBar::scrollButtons_data()