Use accumulator for volume scrolling events

This results in much smoother experience on my touchpad.

src/core/mainwindow.cpp

@@ -1652,7 +1652,7 @@ void MainWindow::PlaylistDoubleClick(const QModelIndex &idx) {
 }
 
 void MainWindow::VolumeWheelEvent(const int delta) {
-  ui_->volume->setValue(ui_->volume->value() + delta / 30);
+  ui_->volume->HandleWheel(delta);
 }
 
 void MainWindow::ToggleShowHide() {

src/widgets/volumeslider.cpp

@@ -54,6 +54,7 @@
 
 VolumeSlider::VolumeSlider(QWidget *parent, const uint max)
     : SliderSlider(Qt::Horizontal, parent, static_cast<int>(max)),
+      wheel_accumulator_(0),
       anim_enter_(false),
       anim_count_(0),
       timer_anim_(new QTimer(this)),
@@ -80,6 +81,23 @@ void VolumeSlider::SetEnabled(const bool enabled) {
   QSlider::setVisible(enabled);
 }
 
+void VolumeSlider::HandleWheel(const int delta) {
+
+  const int scroll_state = wheel_accumulator_ + delta;
+  const int steps = scroll_state / WHEEL_ROTATION_PER_STEP;
+  wheel_accumulator_ = scroll_state % WHEEL_ROTATION_PER_STEP;
+
+  if (steps != 0) {
+    wheeling_ = true;
+
+    QSlider::setValue(SliderSlider::value() + steps);
+    emit SliderReleased(value());
+
+    wheeling_ = false;
+  }
+
+}
+
 void VolumeSlider::paintEvent(QPaintEvent*) {
 
   QPainter p(this);
@@ -272,13 +290,5 @@ void VolumeSlider::mousePressEvent(QMouseEvent *e) {
 }
 
 void VolumeSlider::wheelEvent(QWheelEvent *e) {
-
+  HandleWheel(e->angleDelta().y());
-  wheeling_ = true;
-
-  const int step = e->angleDelta().y() / (e->angleDelta().x() == 0 ? 30 : -30);
-  QSlider::setValue(SliderSlider::value() + step);
-  emit SliderReleased(value());
-
-  wheeling_ = false;
-
 }

src/widgets/volumeslider.h

@@ -48,6 +48,7 @@ class VolumeSlider : public SliderSlider {
  public:
   explicit VolumeSlider(QWidget *parent, uint max = 0);
   void SetEnabled(const bool enabled);
+  void HandleWheel(const int delta);
 
  protected:
 #if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
@@ -70,6 +71,9 @@ class VolumeSlider : public SliderSlider {
   static const int ANIM_INTERVAL = 18;
   static const int ANIM_MAX = 18;
 
+  // Units are eighths of a degree
+  static const int WHEEL_ROTATION_PER_STEP = 30;
+
   VolumeSlider(const VolumeSlider&);
   VolumeSlider &operator=(const VolumeSlider&);
 
@@ -77,6 +81,8 @@ class VolumeSlider : public SliderSlider {
   QPixmap drawVolumePixmap() const;
   void drawVolumeSliderHandle();
 
+  int wheel_accumulator_;
+
   bool anim_enter_;
   int anim_count_;
   QTimer *timer_anim_;