Fix ComposePanel background (#2670)

Author: m-sasha

compose/ui/ui/api/desktop/ui.api

@@ -504,6 +504,7 @@ public final class androidx/compose/ui/awt/ComposePanel : javax/swing/JLayeredPa
 	public fun requestFocus (Z)Z
 	public fun requestFocusInWindow ()Z
 	public fun requestFocusInWindow (Ljava/awt/event/FocusEvent$Cause;)Z
+	public fun setBackground (Ljava/awt/Color;)V
 	public fun setBounds (IIII)V
 	public fun setComponentOrientation (Ljava/awt/ComponentOrientation;)V
 	public final fun setContent (Lkotlin/jvm/functions/Function2;)V

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/awt/ComposePanel.desktop.kt

@@ -90,7 +90,6 @@ class ComposePanel @ExperimentalComposeUiApi constructor(
                 " (use SwingUtilities.invokeLater).\n" +
                 "Creating from another thread isn't supported."
         }
-        background = Color.white
         layout = null
         focusTraversalPolicy = object : FocusTraversalPolicy() {
             override fun getComponentAfter(
@@ -212,6 +211,12 @@ class ComposePanel @ExperimentalComposeUiApi constructor(
         _composeContainer?.preferredSize ?: Dimension(0, 0)
     }
 
+    override fun setBackground(bg: Color?) {
+        // Note that unless `setOpaque(true)` is called, JLayeredPane will not paint the background
+        super.setBackground(bg)
+        _composeContainer?.contentComponent?.background = bg
+    }
+
     /**
      * Sets Compose content of the ComposePanel.
      *
@@ -300,6 +305,7 @@ class ComposePanel @ExperimentalComposeUiApi constructor(
             setBounds(0, 0, width, height)
             contentComponent.isFocusable = isFocusable
             contentComponent.isRequestFocusEnabled = isRequestFocusEnabled
+            contentComponent.background = background
             exceptionHandler = this@ComposePanel.exceptionHandler
 
             _focusListeners.forEach(contentComponent::addFocusListener)

compose/ui/ui/src/desktopMain/kotlin/androidx/compose/ui/scene/skia/WindowSkiaLayerComponent.desktop.kt

@@ -140,16 +140,15 @@ internal class WindowSkiaLayerComponent(
         get() = contentComponent.transparency
         set(value) {
             contentComponent.transparency = value
-            if (value && !windowContext.isWindowTransparent && renderApi == GraphicsApi.METAL) {
-                /*
-                 * SkiaLayer sets background inside transparency setter, that is required for
-                 * cases like software rendering.
-                 * In case of transparent Metal canvas on opaque window, background values with
-                 * alpha == 0 will make the result color black after clearing the canvas.
-                 *
-                 * Reset it to null to keep the color default.
-                 */
-                contentComponent.background = null
+            contentComponent.background = when {
+                !value -> null  // This will use the parent's background
+                // In case of transparent Metal canvas on an opaque window, background values with
+                // alpha == 0 will make the result color black after clearing the canvas.
+                // The case of `transparency = true` together with `!windowContext.isWindowTransparent`
+                // is needed for "Experimental interop on a regular (non-transparent) window"
+                // (per @MatkovIvan)
+                !windowContext.isWindowTransparent && (renderApi == GraphicsApi.METAL) -> null
+                else -> java.awt.Color(0, 0, 0, 0)
             }
         }
     override var fullscreen by contentComponent::fullscreen

compose/ui/ui/src/desktopTest/kotlin/androidx/compose/ui/awt/ComposePanelTest.kt

@@ -897,4 +897,45 @@ class ComposePanelTest {
             window.dispose()
         }
     }
+
+    @Test
+    fun `ComposePanel draws background correctly`() = runApplicationTest {
+        // Show a canvas and a `ComposePanel` with the same background and compare the two colors.
+        // Simply comparing to the set color doesn't work because Robot returns the color after
+        // the OS transforms it to the screen color space (which doesn't seem to be accessible from
+        // the JVM).
+
+        val bgColor = java.awt.Color.RED
+
+        val canvas = java.awt.Canvas().apply {
+            size = Dimension(300, 300)
+            background = bgColor
+        }
+
+        val composePanel = ComposePanel().apply {
+            size = Dimension(300, 300)
+            background = bgColor
+        }
+        composePanel.setContent { }
+
+
+        val frame = JFrame().apply {
+            contentPane.layout = BoxLayout(contentPane, BoxLayout.Y_AXIS)
+            contentPane.add(canvas)
+            contentPane.add(composePanel)
+            size = Dimension(300, 600)
+        }
+
+        try {
+            frame.isVisible = true
+            awaitIdle()
+            val robot = java.awt.Robot()
+            val frameBounds = frame.bounds
+            val canvasPixel = robot.getPixelColor(frameBounds.centerX.toInt(), (0.25 * frameBounds.centerY).toInt())
+            val composePixel = robot.getPixelColor(frameBounds.centerX.toInt(), (0.75 * frameBounds.centerY).toInt())
+            assertThat(composePixel).isEqualTo(canvasPixel)
+        } finally {
+            frame.dispose()
+        }
+    }
 }