set to device layer the same background color as the window have

skiko/src/awtMain/kotlin/org/jetbrains/skiko/SkiaLayer.awt.kt

@@ -135,6 +135,13 @@ actual open class SkiaLayer internal constructor(
                 }
             }
         }
+
+        addPropertyChangeListener("background") { evt ->
+            val color = (evt.newValue as Color).let { color ->
+                org.jetbrains.skia.Color.makeARGB(color.alpha, color.red, color.green, color.blue)
+            }
+            redrawer?.setLayerBackground(color)
+        }
     }
 
     private var fullscreenAdapter = FullscreenAdapter(backedLayer)

skiko/src/awtMain/kotlin/org/jetbrains/skiko/redrawer/MetalRedrawer.kt

@@ -5,6 +5,7 @@ import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.withContext
 import org.jetbrains.skia.BackendRenderTarget
+import org.jetbrains.skia.Color
 import org.jetbrains.skia.DirectContext
 import org.jetbrains.skiko.*
 import org.jetbrains.skiko.context.MetalContextHandler
@@ -39,7 +40,10 @@ internal class MetalRedrawer(
         adapterMemorySize = getAdapterMemorySize(adapter)
         onDeviceChosen(adapterName)
         device = layer.backedLayer.useDrawingSurfacePlatformInfo {
-            createMetalDevice(layer.windowHandle, layer.transparency, adapter, it)
+            val color = layer.background.let { color ->
+                Color.makeARGB(color.alpha, color.red, color.green, color.blue)
+            }
+            createMetalDevice(layer.windowHandle, layer.transparency, color, adapter, it)
         }
     }
 
@@ -140,6 +144,10 @@ internal class MetalRedrawer(
         setLayerVisible(device, isVisible)
     }
 
+    override fun setLayerBackground(color: Int) {
+        setLayerBackground(device, color)
+    }
+
     fun makeContext() = DirectContext(
         makeMetalContext(device)
     )
@@ -151,13 +159,14 @@ internal class MetalRedrawer(
     fun finishFrame() = finishFrame(device)
 
     private external fun chooseAdapter(adapterPriority: Int): Long
-    private external fun createMetalDevice(window:Long, transparency: Boolean, adapter: Long, platformInfo: Long): Long
+    private external fun createMetalDevice(window:Long, transparency: Boolean, color: Int, adapter: Long, platformInfo: Long): Long
     private external fun makeMetalContext(device: Long): Long
     private external fun makeMetalRenderTarget(device: Long, width: Int, height: Int): Long
     private external fun disposeDevice(device: Long)
     private external fun finishFrame(device: Long)
     private external fun resizeLayers(device: Long, x: Int, y: Int, width: Int, height: Int)
     private external fun setLayerVisible(device: Long, isVisible: Boolean)
+    private external fun setLayerBackground(device: Long, color: Int)
     private external fun setContentScale(device: Long, contentScale: Float)
     private external fun setVSyncEnabled(device: Long, enabled: Boolean)
     private external fun isOccluded(window: Long): Boolean

skiko/src/awtMain/objectiveC/macos/MetalRedrawer.mm

@@ -186,8 +186,28 @@ JNIEXPORT jlong JNICALL Java_org_jetbrains_skiko_redrawer_MetalRedrawer_chooseAd
     }
 }
 
+CGColorRef decodeColor(int color) {
+    CGFloat alpha = ((color >> 24) & 0xFF) / 255.0;
+    CGFloat red   = ((color >> 16) & 0xFF) / 255.0;
+    CGFloat green = ((color >> 8) & 0xFF) / 255.0;
+    CGFloat blue  = (color & 0xFF) / 255.0;
+
+    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
+    CGFloat components[] = {red, green, blue, alpha};
+    CGColorRef result = CGColorCreate(colorSpace, components);
+    CGColorSpaceRelease(colorSpace);
+
+    return result;
+}
+
 JNIEXPORT jlong JNICALL Java_org_jetbrains_skiko_redrawer_MetalRedrawer_createMetalDevice(
-    JNIEnv *env, jobject redrawer, jlong windowPtr, jboolean transparency, jlong adapterPtr, jlong platformInfoPtr)
+    JNIEnv *env,
+    jobject redrawer,
+    jlong windowPtr,
+    jboolean transparency,
+    jint backgroundColor,
+    jlong adapterPtr,
+    jlong platformInfoPtr)
 {
     @autoreleasepool {
         id<MTLDevice> adapter = (__bridge_transfer id<MTLDevice>) (void *) adapterPtr;
@@ -216,14 +236,16 @@ JNIEXPORT jlong JNICALL Java_org_jetbrains_skiko_redrawer_MetalRedrawer_createMe
         device.layer.pixelFormat = MTLPixelFormatBGRA8Unorm;
         device.layer.contentsGravity = kCAGravityTopLeft;
 
-        CGFloat transparent[] = { 0.0f, 0.0f, 0.0f, 0.0f };
-        device.layer.backgroundColor = CGColorCreate(CGColorSpaceCreateDeviceRGB(), transparent);
-        device.layer.opaque = NO;
+        NSWindow* window = (__bridge NSWindow*) (void *) windowPtr;
 
+        device.layer.opaque = NO;
         if (transparency)
         {
-            NSWindow* window = (__bridge NSWindow*) (void *) windowPtr;
+            CGFloat transparent[] = { 0.0f, 0.0f, 0.0f, 0.0f };
+            device.layer.backgroundColor = CGColorCreate(CGColorSpaceCreateDeviceRGB(), transparent);
             window.hasShadow = NO;
+        } else {
+            device.layer.backgroundColor = decodeColor(backgroundColor);
         }
 
         return (jlong) (__bridge_retained void *) device;
@@ -268,6 +290,18 @@ JNIEXPORT void JNICALL Java_org_jetbrains_skiko_redrawer_MetalRedrawer_setLayerV
     }
 }
 
+JNIEXPORT void JNICALL Java_org_jetbrains_skiko_redrawer_MetalRedrawer_setLayerBackground(
+    JNIEnv *env, jobject redrawer, jlong devicePtr, jint color)
+{
+    @autoreleasepool {
+        MetalDevice *device = (__bridge MetalDevice *) (void *) devicePtr;
+        if (!device || !device.layer) {
+            return;
+        }
+        device.layer.backgroundColor = decodeColor(color);
+    }
+}
+
 JNIEXPORT void JNICALL Java_org_jetbrains_skiko_redrawer_MetalRedrawer_setContentScale(JNIEnv *env, jobject obj, jlong devicePtr, jfloat contentScale)
 {
     @autoreleasepool {

skiko/src/commonMain/kotlin/org/jetbrains/skiko/redrawer/Redrawer.kt

@@ -6,5 +6,7 @@ internal interface Redrawer {
     fun redrawImmediately()
     fun syncSize() = Unit
     fun setVisible(isVisible: Boolean) = Unit
+    // ARGB encoded as in org.jetbrains.skia.Color
+    fun setLayerBackground(color: Int) = Unit
     val renderInfo: String
 }