ComboBox: fixed popup border painting on HiDPI screens (e.g. at 150% scaling)

CHANGELOG.md

@@ -20,6 +20,7 @@ FlatLaf Change Log
 - OptionPane: Fixed rendering of longer HTML text if it is passed as
   `StringBuilder`, `StringBuffer`, or any other object that returns HTML text in
   method `toString()`. (similar to issue #12)
+- ComboBox: Fixed popup border painting on HiDPI screens (e.g. at 150% scaling).
 - ComboBox (editable): Fixed wrong border of internal text field under special
   circumstances.
 - Spinner: Fixed painting of border corners on left side. (issue #382;

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

@@ -695,6 +695,9 @@ public class FlatComboBoxUI
 		protected void configurePopup() {
 			super.configurePopup();
 
+			// make opaque to avoid that background shines thru border (e.g. at 150% scaling)
+			setOpaque( true );
+
 			Border border = UIManager.getBorder( "PopupMenu.border" );
 			if( border != null )
 				setBorder( border );