Add configurable clearColor to SkiaLayer to avoid white background during resize on Windows.

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

@@ -30,6 +30,7 @@ actual open class SkiaLayer internal constructor(
     private val analytics: SkiaLayerAnalytics = SkiaLayerAnalytics.Empty,
     actual val pixelGeometry: PixelGeometry = PixelGeometry.UNKNOWN,
 ) : JPanel() {
+    actual val clearColor: Int = properties.clearColor
 
     internal companion object {
         init {

skiko/src/commonMain/kotlin/org/jetbrains/skiko/SkiaLayer.kt

@@ -43,6 +43,11 @@ expect open class SkiaLayer {
      */
     var renderDelegate: SkikoRenderDelegate?
 
+    /**
+     * Clear color for the canvas.
+     */
+    val clearColor: Int
+
     /**
      * Attach this [SkikoRenderDelegate] to platform container.
      * Actual type of attach container is platform-specific.

skiko/src/commonMain/kotlin/org/jetbrains/skiko/context/ContextHandler.kt

@@ -41,7 +41,7 @@ internal abstract class ContextHandler(
         }
         initCanvas()
         canvas?.apply {
-            clear(if (isTransparentBackground()) Color.TRANSPARENT else Color.WHITE)
+            clear(if (isTransparentBackground()) Color.TRANSPARENT else layer.clearColor)
             drawContent()
         }
         flush()

skiko/src/jsWasmMain/kotlin/org/jetbrains/skiko/SkiaLayer.js.kt

@@ -45,6 +45,11 @@ actual open class SkiaLayer {
             if (value) throw Exception("Transparency is not supported!")
         }
 
+    /**
+     * Clear color for the canvas.
+     */
+    actual val clearColor: Int = org.jetbrains.skia.Color.WHITE
+
     /**
      * Schedules a drawFrame to the appropriate moment.
      */

skiko/src/jvmMain/kotlin/org/jetbrains/skiko/SkiaLayerProperties.kt

@@ -22,6 +22,8 @@ class SkiaLayerProperties(
     val renderApi: GraphicsApi = SkikoProperties.renderApi,
     val adapterPriority: GpuPriority = SkikoProperties.gpuPriority,
 ) {
+    val clearColor: Int = clearColorFromSystem()
+
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
 
@@ -56,4 +58,21 @@ class SkiaLayerProperties(
         result = 31 * result + adapterPriority.hashCode()
         return result
     }
+
+    companion object {
+        var defaultClearColor: Int = org.jetbrains.skia.Color.WHITE
+
+        fun clearColorFromSystem(): Int {
+            val colorFromEnv = System.getProperty("SKIKO_CLEAR_COLOR")?.let {
+                try {
+                    if (it[0] == '#') {
+                        it.drop(1).toInt(16)
+                    } else {
+                        it.toInt()
+                    }
+                } catch (_: Exception) { null }
+            }
+            return colorFromEnv ?: defaultClearColor
+        }
+    }
 }

skiko/src/linuxMain/kotlin/org/jetbrains/skiko/SkiaLayer.linux.kt

@@ -15,6 +15,8 @@ actual open class SkiaLayer  {
     actual var transparency: Boolean
         get() = TODO("Not yet implemented")
         set(value) {}
+    actual val clearColor: Int
+        get() = TODO("Not yet implemented")
     actual val component: Any?
         get() = TODO("Not yet implemented")
     actual fun needRedraw() {

skiko/src/macosMain/kotlin/org/jetbrains/skiko/SkiaLayer.macos.kt

@@ -33,6 +33,8 @@ actual open class SkiaLayer {
             field = value
         }
 
+    actual val clearColor: Int = Color.WHITE
+
     /**
      * The scale factor of [NSWindow]
      * https://developer.apple.com/documentation/appkit/nswindow/1419459-backingscalefactor

skiko/src/uikitMain/kotlin/org/jetbrains/skiko/SkiaLayer.uikit.kt

@@ -2,6 +2,7 @@ package org.jetbrains.skiko
 
 import kotlinx.cinterop.useContents
 import org.jetbrains.skia.Canvas
+import org.jetbrains.skia.Color
 import org.jetbrains.skia.PixelGeometry
 import org.jetbrains.skia.Surface
 import platform.UIKit.*
@@ -25,6 +26,8 @@ actual open class SkiaLayer {
         get() = false
         set(_) { throw UnsupportedOperationException() }
 
+    actual val clearColor: Int = Color.WHITE
+
     actual fun needRedraw() {
         needRedrawCallback.invoke()
     }