Add black background default to FlutterView root layer

Author: a-wallen

shell/platform/darwin/macos/framework/Source/FlutterView.h

@@ -22,6 +22,8 @@
  */
 @interface FlutterView : NSView
 
+@property(weak, nonatomic) NSColor* __nullable backgroundColor;
+
 /**
  * Initialize a FlutterView that will be rendered to using Metal rendering apis.
  */

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

@@ -28,6 +28,7 @@ - (instancetype)initWithMTLDevice:(id<MTLDevice>)device
   self = [super initWithFrame:NSZeroRect];
   if (self) {
     [self setWantsLayer:YES];
+    [self paintContainingLayerBackgroundColor];
     [self setLayerContentsRedrawPolicy:NSViewLayerContentsRedrawDuringViewResize];
     _reshapeListener = reshapeListener;
     _resizableBackingStoreProvider =
@@ -51,6 +52,7 @@ - (instancetype)initWithFrame:(NSRect)frame
   self = [super initWithFrame:frame];
   if (self) {
     [self setWantsLayer:YES];
+    [self paintContainingLayerBackgroundColor];
     _reshapeListener = reshapeListener;
     _resizableBackingStoreProvider =
         [[FlutterOpenGLResizableBackingStoreProvider alloc] initWithMainContext:mainContext
@@ -84,6 +86,17 @@ - (void)reshaped {
                             }];
 }
 
+- (NSColor*)backgroundColor {
+  return _backgroundColor ? _backgroundColor : [NSColor blackColor];
+}
+
+- (void)paintContainingLayerBackgroundColor {
+  if (self) {
+    [self setWantsLayer:YES];
+    self.layer.backgroundColor = self.backgroundColor.CGColor;
+  }
+}
+
 #pragma mark - NSView overrides
 
 - (void)setFrameSize:(NSSize)newSize {