Send scroll events from the macOS shell

shell/platform/darwin/macos/framework/Source/FLEViewController.mm

@@ -39,6 +39,14 @@ @interface FLEViewController ()
  */
 @property(nonatomic) NSTrackingArea* trackingArea;
 
+/**
+ * Whether or not a kAdd event has been sent for the mouse (or sent again since
+ * the last kRemove was sent if tracking is enabled). Used to determine whether
+ * to send an Add event before sending an incoming mouse event, since Flutter
+ * expects a pointers to be added before events are sent for them.
+ */
+@property(nonatomic) BOOL mouseCurrentlyAdded;
+
 /**
  * Updates |trackingArea| for the current tracking settings, creating it with
  * the correct mode if tracking is enabled, or removing it if not.
@@ -416,16 +424,54 @@ - (void)handlePlatformMessage:(const FlutterPlatformMessage*)message {
 }
 
 - (void)dispatchMouseEvent:(NSEvent*)event phase:(FlutterPointerPhase)phase {
+  // If a pointer added event hasn't been sent, synthesize one using this event for the basic
+  // information.
+  if (!_mouseCurrentlyAdded && phase != kAdd) {
+    // Only the values extracted for use in flutterEvent below matter, the rest are dummy values.
+    NSEvent* addEvent = [NSEvent enterExitEventWithType:NSEventTypeMouseEntered
+                                               location:event.locationInWindow
+                                          modifierFlags:0
+                                              timestamp:event.timestamp
+                                           windowNumber:event.windowNumber
+                                                context:nil
+                                            eventNumber:0
+                                         trackingNumber:0
+                                               userData:NULL];
+    [self dispatchMouseEvent:addEvent phase:kAdd];
+  }
+
   NSPoint locationInView = [self.view convertPoint:event.locationInWindow fromView:nil];
   NSPoint locationInBackingCoordinates = [self.view convertPointToBacking:locationInView];
-  const FlutterPointerEvent flutterEvent = {
+  FlutterPointerEvent flutterEvent = {
       .struct_size = sizeof(flutterEvent),
       .phase = phase,
       .x = locationInBackingCoordinates.x,
       .y = -locationInBackingCoordinates.y,  // convertPointToBacking makes this negative.
       .timestamp = static_cast<size_t>(event.timestamp * NSEC_PER_MSEC),
   };
+
+  if (event.type == NSEventTypeScrollWheel) {
+    flutterEvent.signal_kind = kFlutterPointerSignalKindScroll;
+
+    double pixelsPerLine = 1.0;
+    if (!event.hasPreciseScrollingDeltas) {
+      CGEventSourceRef source = CGEventCreateSourceFromEvent(event.CGEvent);
+      pixelsPerLine = CGEventSourceGetPixelsPerLine(source);
+      if (source) {
+        CFRelease(source);
+      }
+    }
+    double scaleFactor = self.view.layer.contentsScale;
+    flutterEvent.scroll_delta_x = event.scrollingDeltaX * pixelsPerLine * scaleFactor;
+    flutterEvent.scroll_delta_y = -event.scrollingDeltaY * pixelsPerLine * scaleFactor;
+  }
   FlutterEngineSendPointerEvent(_engine, &flutterEvent, 1);
+
+  if (phase == kAdd) {
+    _mouseCurrentlyAdded = YES;
+  } else if (phase == kRemove) {
+    _mouseCurrentlyAdded = NO;
+  }
 }
 
 - (void)dispatchKeyEvent:(NSEvent*)event ofType:(NSString*)type {
@@ -539,4 +585,10 @@ - (void)mouseMoved:(NSEvent*)event {
   [self dispatchMouseEvent:event phase:kHover];
 }
 
+- (void)scrollWheel:(NSEvent*)event {
+  // TODO: Add gesture-based (trackpad) scroll support once it's supported by the engine rather
+  // than always using kHover.
+  [self dispatchMouseEvent:event phase:kHover];
+}
+
 @end