From c45a769aa36676476f47b9e6b7f189715298709d Mon Sep 17 00:00:00 2001
From: Karl Tauber <karl@jformdesigner.com>
Date: Tue, 2 Jun 2020 15:46:36 +0200
Subject: [PATCH] update JFrame/JDialog background color when switching Laf

---
 .../formdev/flatlaf/ui/FlatRootPaneUI.java    | 24 +++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java
index 1f473487..f09b560e 100644
--- a/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java
+++ b/flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRootPaneUI.java
@@ -16,8 +16,15 @@
 
 package com.formdev.flatlaf.ui;
 
+import java.awt.Color;
+import java.awt.Container;
 import javax.swing.JComponent;
+import javax.swing.JDialog;
+import javax.swing.JFrame;
+import javax.swing.JRootPane;
+import javax.swing.UIManager;
 import javax.swing.plaf.ComponentUI;
+import javax.swing.plaf.UIResource;
 import javax.swing.plaf.basic.BasicRootPaneUI;
 
 /**
@@ -35,4 +42,21 @@ public class FlatRootPaneUI
 			instance = new FlatRootPaneUI();
 		return instance;
 	}
+
+	@Override
+	protected void installDefaults( JRootPane c ) {
+		super.installDefaults( c );
+
+		// Update background color of JFrame or JDialog parent to avoid bad border
+		// on HiDPI screens when switching from light to dark Laf.
+		// The background of JFrame is initialized in JFrame.frameInit() and
+		// the background of JDialog in JDialog.dialogInit(),
+		// but it was not updated when switching Laf.
+		Container parent = c.getParent();
+		if( parent instanceof JFrame || parent instanceof JDialog ) {
+			Color background = parent.getBackground();
+			if( background == null || background instanceof UIResource )
+				parent.setBackground( UIManager.getColor( "control" ) );
+		}
+	}
 }