Main thread is blocked when rendering for Playground app on macOS and iOS

[bghgary]

The playground app on macOS and iOS:

BabylonNative/Apps/Playground/iOS/LibNativeBridge.mm

Lines 105 to 112 in a02f821

	if (graphics)
	{
	graphics->StartRenderingCurrentFrame();
	update->Start();
	update->Finish();
	graphics->FinishRenderingCurrentFrame();
	}
	}

BabylonNative/Apps/Playground/macOS/ViewController.mm

Lines 39 to 44 in 76e5ba1

	if (graphics) {
	graphics->StartRenderingCurrentFrame();
	update->Start();
	update->Finish();
	graphics->FinishRenderingCurrentFrame();
	}

The code is currently calling FinishRenderingCurrentFrame immediately following StartRenderingCurrentFrame. FinishedRenderingCurrentFrame will immediately block waiting for the JS thread to finish doing its part. Ideally, these calls should be backwards. StartRenderingCurrentFrame should be called as soon as possible and FinishRenderingCurrentFrame should be called as late as possible. There should be a small gap between FinishRenderingCurrentFrame and the next StartRenderingCurrentFrame.

Oddly, the validation tests for macOS and iOS does it correctly:

BabylonNative/Apps/ValidationTests/iOS/LibNativeBridge.mm

Lines 87 to 93 in a02f821

	if (graphics)
	{
	update->Finish();
	graphics->FinishRenderingCurrentFrame();
	graphics->StartRenderingCurrentFrame();
	update->Start();
	}

BabylonNative/Apps/ValidationTests/macOS/ViewController.mm

Lines 36 to 41 in a02f821

	if (graphics) {
	update->Finish();
	graphics->FinishRenderingCurrentFrame();
	graphics->StartRenderingCurrentFrame();
	update->Start();
	}

[bghgary]

Related: BabylonJS/BabylonReactNative#343

[CedricGuillemet]

This is a resize issue because RenderTarget and other resources are still being used while resize occurs.
Issue doesn't appear with VT because there is no resize allowed there.

[thomlucc]

fixed