OptionPane: do not make child components, which are derived from JPanel, non-opaque (issue #349)

CHANGELOG.md

@@ -28,6 +28,8 @@ FlatLaf Change Log
   components by:
   - `2px` at `1.75x` in **Light** and **Dark** themes
   - `2px` at `1.25x` and `2.25x` in **IntelliJ** and **Darcula** themes
+- OptionPane: Do not make child components, which are derived from `JPanel`,
+  non-opaque. (issue #349)
 
 
 ## 1.3

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

@@ -165,7 +165,7 @@ public class FlatOptionPaneUI
 
 	private void updateChildPanels( Container c ) {
 		for( Component child : c.getComponents() ) {
-			if( child instanceof JPanel ) {
+			if( child.getClass() == JPanel.class ) {
 				JPanel panel = (JPanel)child;
 
 				// make sub-panel non-opaque for OptionPane.background
@@ -177,9 +177,8 @@ public class FlatOptionPaneUI
 					panel.setBorder( new NonUIResourceBorder( border ) );
 			}
 
-			if( child instanceof Container ) {
+			if( child instanceof Container )
 				updateChildPanels( (Container) child );
-			}
 		}
 	}