Undo axes flip on Mac when shift+scroll-wheel

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

@@ -643,8 +643,23 @@ - (void)dispatchMouseEvent:(NSEvent*)event phase:(FlutterPointerPhase)phase {
       pixelsPerLine = 40.0;
     }
     double scaleFactor = self.flutterView.layer.contentsScale;
-    flutterEvent.scroll_delta_x = -event.scrollingDeltaX * pixelsPerLine * scaleFactor;
-    flutterEvent.scroll_delta_y = -event.scrollingDeltaY * pixelsPerLine * scaleFactor;
+    // When mouse input is received while shift is pressed (regardless of
+    // any other pressed keys), Mac automatically flips the axis. Other
+    // platforms do not do this, so we flip it back to normalize the input
+    // received by the framework. The keyboard+mouse-scroll mechanism is exposed
+    // in the ScrollBehavior of the framework so developers can customize the
+    // behavior.
+    // At time of change, Apple does not expose any other type of API or signal
+    // that the X/Y axes have been flipped.
+    double scaledDeltaX = -event.scrollingDeltaX * pixelsPerLine * scaleFactor;
+    double scaledDeltaY = -event.scrollingDeltaY * pixelsPerLine * scaleFactor;
+    if (event.modifierFlags & NSShiftKeyMask) {
+      flutterEvent.scroll_delta_x = scaledDeltaY;
+      flutterEvent.scroll_delta_y = scaledDeltaX;
+    } else {
+      flutterEvent.scroll_delta_x = scaledDeltaX;
+      flutterEvent.scroll_delta_y = scaledDeltaY;
+    }
   }
   [_engine sendPointerEvent:flutterEvent];
 

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

@@ -629,6 +629,27 @@ - (bool)testTrackpadGesturesAreSentToFramework {
   EXPECT_EQ(last_event.scroll_delta_x, -40 * viewController.flutterView.layer.contentsScale);
   EXPECT_EQ(last_event.scroll_delta_y, -80 * viewController.flutterView.layer.contentsScale);
 
+  // A discrete scroll event should use the PointerSignal system, and flip the
+  // direction when shift is pressed.
+  CGEventRef cgEventDiscreteShift =
+      CGEventCreateScrollWheelEvent(NULL, kCGScrollEventUnitPixel, 1, 0);
+  CGEventSetType(cgEventDiscreteShift, kCGEventScrollWheel);
+  CGEventSetFlags(cgEventDiscreteShift, kCGEventFlagMaskShift);
+  CGEventSetIntegerValueField(cgEventDiscreteShift, kCGScrollWheelEventIsContinuous, 0);
+  CGEventSetIntegerValueField(cgEventDiscreteShift, kCGScrollWheelEventDeltaAxis2,
+                              0);  // scroll_delta_x
+  CGEventSetIntegerValueField(cgEventDiscreteShift, kCGScrollWheelEventDeltaAxis1,
+                              2);  // scroll_delta_y
+
+  called = false;
+  [viewController scrollWheel:[NSEvent eventWithCGEvent:cgEventDiscreteShift]];
+  EXPECT_TRUE(called);
+  EXPECT_EQ(last_event.signal_kind, kFlutterPointerSignalKindScroll);
+  // pixelsPerLine is 40.0, direction is reversed and axes have been flipped back.
+  EXPECT_FLOAT_EQ(last_event.scroll_delta_x, 0.0 * viewController.flutterView.layer.contentsScale);
+  EXPECT_FLOAT_EQ(last_event.scroll_delta_y,
+                  -80.0 * viewController.flutterView.layer.contentsScale);
+
   // Test for scale events.
   // Start gesture.
   called = false;