InternalFrame: fill background to avoid that parent may shine through internal frame if it contains non-opaque components (better fix for issue #274)

CHANGELOG.md

@@ -90,6 +90,12 @@ FlatLaf Change Log
   - New "column control" icon for `JXTable` that scales and uses antialiasing.
     (issue #434)
 
+#### Fixed bugs
+
+- InternalFrame: Fill background to avoid that parent may shine through internal
+  frame if it contains non-opaque components. (better fix for issue #274)
+
+
 ## 1.6.5
 
 #### Fixed bugs

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatInternalFrameTitlePane.java

@@ -213,6 +213,13 @@ public class FlatInternalFrameTitlePane
 				case "componentOrientation":
 					applyComponentOrientation( frame.getComponentOrientation() );
 					break;
+
+				case "opaque":
+					// Do not invoke super.propertyChange() here because it always
+					// invokes repaint(), which would cause endless repainting.
+					// The opaque flag is temporary changed in FlatUIUtils.hasOpaqueBeenExplicitlySet(),
+					// invoked from FlatInternalFrameUI.update().
+					return;
 			}
 
 			super.propertyChange( e );

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatInternalFrameUI.java

@@ -179,6 +179,26 @@ public class FlatInternalFrameUI
 		return FlatStylingSupport.getAnnotatedStyleableInfos( this, frame.getBorder() );
 	}
 
+	@Override
+	public void update( Graphics g, JComponent c ) {
+		// The internal frame actually should be opaque and fill its background,
+		// but it must be non-opaque to allow translucent resize handles (outside of visual bounds).
+		// To avoid that parent may shine through internal frame (e.g. if menu bar is non-opaque),
+		// fill background excluding insets (translucent resize handles),
+		// but only if opaque was not set explicitly by application to false.
+		// If applications has set internal frame opacity to false, do not fill background (for compatibility).
+		if( !c.isOpaque() && !FlatUIUtils.hasOpaqueBeenExplicitlySet( c ) ) {
+			Insets insets = c.getInsets();
+
+			g.setColor( c.getBackground() );
+			g.fillRect( insets.left, insets.top,
+				c.getWidth() - insets.left - insets.right,
+				c.getHeight() - insets.top - insets.bottom );
+		}
+
+		super.update( g, c );
+	}
+
 	//---- class FlatInternalFrameBorder --------------------------------------
 
 	public static class FlatInternalFrameBorder