macOS: render the map at the window backing scale

platform/macos/src/MLNMapView+Impl.h

@@ -22,6 +22,8 @@ class MLNMapViewImpl : public mbgl::MapObserver {
 
   virtual CGLContextObj getCGLContextObj() { return nullptr; }
 
+  virtual void backingPropertiesChanged() {}
+
 #if MLN_RENDER_BACKEND_METAL
   // Returns the backend resource for Metal rendering in custom layers
   virtual MLNBackendResource* getObject() { return nullptr; }

platform/macos/src/MLNMapView+Metal.h

@@ -38,6 +38,8 @@ class MLNMapViewMetalImpl final : public MLNMapViewImpl,
   mbgl::PremultipliedImage readStillImage() override;
   MLNBackendResource* getObject() override;
 
+  void backingPropertiesChanged() override;
+
 private:
   bool presentsWithTransaction = false;
 };

platform/macos/src/MLNMapView+Metal.mm

@@ -129,6 +129,18 @@ void swap() override {
   [mapView addSubview:resource.mtlView positioned:NSWindowBelow relativeTo:nil];
 }
 
+void MLNMapViewMetalImpl::backingPropertiesChanged() {
+  auto& resource = getResource<MLNMapViewMetalRenderableResource>();
+  const CGFloat scale = mapView.window.backingScaleFactor;
+  if (!resource.mtlView || scale <= 0) {
+    return;
+  }
+  resource.mtlView.layer.contentsScale = scale;
+  const CGSize bounds = resource.mtlView.bounds.size;
+  resource.mtlView.drawableSize =
+      CGSizeMake(std::round(bounds.width * scale), std::round(bounds.height * scale));
+}
+
 MLNMapViewMetalImpl::~MLNMapViewMetalImpl() = default;
 
 void MLNMapViewMetalImpl::activate() {

platform/macos/src/MLNMapView.mm

@@ -807,6 +807,10 @@ - (void)viewDidMoveToWindow {
     self.dormant = NO;
   }
 
+  if (window && _mbglView) {
+    _mbglView->backingPropertiesChanged();
+  }
+
   if (window && _mbglMap->getMapOptions().constrainMode() == mbgl::ConstrainMode::None) {
     _mbglMap->setConstrainMode(mbgl::ConstrainMode::HeightOnly);
   }
@@ -821,6 +825,13 @@ - (void)viewDidMoveToWindow {
               context:NULL];
 }
 
+- (void)viewDidChangeBackingProperties {
+  [super viewDidChangeBackingProperties];
+  if (_mbglView) {
+    _mbglView->backingPropertiesChanged();
+  }
+}
+
 - (BOOL)wantsLayer {
   return YES;
 }