macOS: Setting the drop action doesn't work when pressing modifier keys

Because Cocoa will filter the dragging operations depending on which modifier key is pressed, the value returned by -[QNSView draggingSourceOperationMaskForLocal:] may be overridden before we get a chance of reading it in any of the dragging event handlers.

Run the attached example and see what happens when you press the Cmd key while dragging. Notice that the drop action is always set to CopyAction by the application, and so it should show.

It's been reported that things work as expected on Windows.