curve/timeline: improved user interface behavior

* Implements "auto-fit" setting for curve view and sets it on open.
* Improves scroll zooming on all TimelineViewBase derivatives.
* Improves code sharing for better maintenance.

app/widget/timelinewidget/view/timelineviewbase.cpp

@@ -40,7 +40,9 @@ TimelineViewBase::TimelineViewBase(QWidget *parent) :
   playhead_scene_right_(-1),
   dragging_playhead_(false),
   snapped_(false),
-  snap_service_(nullptr)
+  snap_service_(nullptr),
+  y_axis_enabled_(false),
+  y_scale_(1.0)
 {
   setScene(&scene_);
 
@@ -80,6 +82,26 @@ void TimelineViewBase::SetSnapService(SnapService *service)
   snap_service_ = service;
 }
 
+const double &TimelineViewBase::GetYScale() const
+{
+  return y_scale_;
+}
+
+void TimelineViewBase::VerticalScaleChangedEvent(double)
+{
+}
+
+void TimelineViewBase::SetYScale(const double &y_scale)
+{
+  y_scale_ = y_scale;
+
+  if (y_axis_enabled_) {
+    VerticalScaleChangedEvent(y_scale_);
+
+    viewport()->update();
+  }
+}
+
 void TimelineViewBase::SetTime(const int64_t time)
 {
   playhead_ = time;
@@ -252,10 +274,56 @@ bool TimelineViewBase::HandleZoomFromScroll(QWheelEvent *event)
   if (WheelEventIsAZoomEvent(event)) {
     // If CTRL is held (or a preference is set to swap CTRL behavior), we zoom instead of scrolling
     if (!event->angleDelta().isNull()) {
-      if (event->angleDelta().x() + event->angleDelta().y() > 0) {
-        emit ScaleChanged(GetScale() * 1.1);
+      bool only_vertical = false;
+      bool only_horizontal = false;
+
+      // Ctrl+Shift limits to only one axis
+      // Alt switches between horizontal only (alt held) or vertical only (alt not held)
+      if (y_axis_enabled_) {
+        if (event->modifiers() & Qt::ShiftModifier) {
+          if (event->modifiers() & Qt::AltModifier) {
+            only_horizontal = true;
+          } else {
+            only_vertical = true;
+          }
+        }
       } else {
-        emit ScaleChanged(GetScale() * 0.9);
+        only_horizontal = true;
+      }
+
+      double scale_multiplier;
+
+      if (event->angleDelta().x() + event->angleDelta().y() > 0) {
+        scale_multiplier = 1.1;
+      } else {
+        scale_multiplier = 0.9;
+      }
+
+      QPointF cursor_pos;
+#if QT_VERSION >= QT_VERSION_CHECK(5, 14, 0)
+      cursor_pos = event->position();
+#else
+      cursor_pos = event->posF();
+#endif
+
+      if (!only_vertical) {
+        double new_x_scale = GetScale() * scale_multiplier;
+
+        int new_x_scroll = qRound(horizontalScrollBar()->value() / GetScale() * new_x_scale + (cursor_pos.x() - cursor_pos.x() / new_x_scale * GetScale()));
+
+        emit ScaleChanged(new_x_scale);
+
+        horizontalScrollBar()->setValue(new_x_scroll);
+      }
+
+      if (!only_horizontal) {
+        double new_y_scale = GetYScale() * scale_multiplier;
+
+        int new_y_scroll = qRound(verticalScrollBar()->value() / GetYScale() * new_y_scale + (cursor_pos.y() - cursor_pos.y() / new_y_scale * GetYScale()));
+
+        SetYScale(new_y_scale);
+
+        verticalScrollBar()->setValue(new_y_scroll);
       }
     }