Send scroll events from the macOS shell (#8056)

Adds scroll event handling to the macOS shell, sending them using the
new embedding API.

Currently, trackpad scrolling is just handled like scrollwheel
scrolling, since the trackpad-gesture-based scroll handling in Flutter
is still in progress.

Adds code to synthesize Add events if mouse tracking isn't enabled,
so that it doesn't rely on Flutter being lenient about handling events
from pointers that have never been added.

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