[Impeller] Update CAMetalLayer to be opaque

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

@@ -72,13 +72,18 @@ - (instancetype)initWithDelegate:(id<FlutterViewEngineDelegate>)delegate
       FML_DLOG(WARNING) << "Rendering wide gamut colors is turned on but isn't "
                            "supported, downgrading the color gamut to sRGB.";
     }
-    self.layer.opaque = opaque;
 
     // This line is necessary. CoreAnimation(or UIKit) may take this to do
     // something to compute the final frame presented on screen, if we don't set this,
     // it will make it take long time for us to take next CAMetalDrawable and will
-    // cause constant junk during rendering.
+    // cause constant junk during rendering. Note: setting any background color will
+    // disable opaque, which on full screen flutter apps removes the direct to display
+    // optimization. This can increase presentation time by a frame interval, which
+    // is problematic on high frame rate devices.
     self.backgroundColor = UIColor.clearColor;
+    // Make sure to set this value after backgroundColor above, otherwise it will
+    // reset the value of opaque.
+    self.layer.opaque = opaque;
   }
 
   return self;
@@ -96,7 +101,7 @@ - (void)layoutSubviews {
     layer.allowsGroupOpacity = YES;
     layer.contentsScale = screenScale;
     layer.rasterizationScale = screenScale;
-    layer.framebufferOnly = flutter::Settings::kSurfaceDataAccessible ? NO : YES;
+    layer.framebufferOnly = NO;
     if (_isWideGamutEnabled && self.isWideGamutSupported) {
       CGColorSpaceRef srgb = CGColorSpaceCreateWithName(kCGColorSpaceExtendedSRGB);
       layer.colorspace = srgb;

shell/platform/darwin/ios/framework/Source/FlutterViewTest.mm

@@ -69,4 +69,26 @@ - (void)testIgnoreWideColorWithoutImpeller {
   XCTAssertEqual(layer.pixelFormat, MTLPixelFormatBGRA8Unorm);
 }
 
+- (void)testLayerOpacityIsPreservedYes {
+  FakeDelegate* delegate = [[[FakeDelegate alloc] init] autorelease];
+  FlutterView* view = [[[FlutterView alloc] initWithDelegate:delegate opaque:YES
+                                             enableWideGamut:NO] autorelease];
+
+  XCTAssertTrue(view.layer.opaque);
+  CGColor* bgColor = view.layer.backgroundColor;
+  CGColor* expectedColor = [UIColor.clearColor CGColor];
+  XCTAssertEqual(bgColor, expectedColor);
+}
+
+- (void)testLayerOpacityIsPreservedNo {
+  FakeDelegate* delegate = [[[FakeDelegate alloc] init] autorelease];
+  FlutterView* view = [[[FlutterView alloc] initWithDelegate:delegate opaque:NO
+                                             enableWideGamut:NO] autorelease];
+
+  XCTAssertFalse(view.layer.opaque);
+  CGColor* bgColor = view.layer.backgroundColor;
+  CGColor* expectedColor = [UIColor.clearColor CGColor];
+  XCTAssertEqual(bgColor, expectedColor);
+}
+
 @end