Linux: support using custom window decorations (issue #482)

CHANGELOG.md

@@ -1,6 +1,16 @@
 FlatLaf Change Log
 ==================
 
+## 2.1-SNAPSHOT
+
+#### New features and improvements
+
+- Linux: Support using custom window decorations. Enable with
+  `JFrame.setDefaultLookAndFeelDecorated(true)` and
+  `JDialog.setDefaultLookAndFeelDecorated(true)` before creating a window.
+  (issue #482)
+
+
 ## 2.0.2
 
 - Native window decorations (Windows 10/11 only): Fixed rendering artifacts on

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -166,11 +166,12 @@ public abstract class FlatLaf
 	 * Returns whether FlatLaf supports custom window decorations.
 	 * This depends on the operating system and on the used Java runtime.
 	 * <p>
-	 * This method returns {@code true} on Windows 10 (see exception below), {@code false} otherwise.
+	 * This method returns {@code true} on Windows 10/11 (see exception below)
+	 * and on Linux, {@code false} otherwise.
 	 * <p>
-	 * Returns also {@code false} on Windows 10 if:
+	 * Returns also {@code false} on Windows 10/11 if:
 	 * <ul>
-	 * <li>FlatLaf native window border support is available (requires Windows 10)</li>
+	 * <li>FlatLaf native window border support is available (requires Windows 10/11)</li>
 	 * <li>running in
 	 * <a href="https://confluence.jetbrains.com/display/JBR/JetBrains+Runtime">JetBrains Runtime 11 (or later)</a>
 	 * (<a href="https://github.com/JetBrains/JetBrainsRuntime">source code on github</a>)
@@ -190,7 +191,7 @@ public abstract class FlatLaf
 			FlatNativeWindowBorder.isSupported() )
 		  return false;
 
-		return SystemInfo.isWindows_10_orLater;
+		return SystemInfo.isWindows_10_orLater || SystemInfo.isLinux;
 	}
 
 	@Override
@@ -995,7 +996,7 @@ public abstract class FlatLaf
 	/**
 	 * Returns whether native window decorations are supported on current platform.
 	 * <p>
-	 * This requires Windows 10, but may be disabled if running in special environments
+	 * This requires Windows 10/11, but may be disabled if running in special environments
 	 * (JetBrains Projector, Webswing or WinPE) or if loading native library fails.
 	 * If system property {@link FlatSystemProperties#USE_WINDOW_DECORATIONS} is set to
 	 * {@code false}, then this method also returns {@code false}.

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

@@ -210,7 +210,7 @@ public class FlatMenuBarUI
 		// check whether:
 		// - TitlePane.unifiedBackground is true and
 		// - menu bar is the "main" menu bar and
-		// - window has custom decorations enabled
+		// - window root pane has custom decoration style
 
 		JRootPane rootPane;
 		// (not storing value of "TitlePane.unifiedBackground" in class to allow changing at runtime)
@@ -218,7 +218,7 @@ public class FlatMenuBarUI
 			(rootPane = SwingUtilities.getRootPane( c )) != null &&
 			rootPane.getParent() instanceof Window &&
 			rootPane.getJMenuBar() == c &&
-			FlatNativeWindowBorder.hasCustomDecoration( (Window) rootPane.getParent() );
+			rootPane.getWindowDecorationStyle() != JRootPane.NONE;
 	}
 
 	//---- class TakeFocus ----------------------------------------------------

flatlaf-demo/src/main/java/com/formdev/flatlaf/demo/DemoFrame.java

@@ -889,8 +889,11 @@ class DemoFrame
 		copyMenuItem.addActionListener( new DefaultEditorKit.CopyAction() );
 		pasteMenuItem.addActionListener( new DefaultEditorKit.PasteAction() );
 
-		if( FlatLaf.supportsNativeWindowDecorations() ) {
-			windowDecorationsCheckBoxMenuItem.setSelected( FlatLaf.isUseNativeWindowDecorations() );
+		if( FlatLaf.supportsNativeWindowDecorations() || (SystemInfo.isLinux && JFrame.isDefaultLookAndFeelDecorated()) ) {
+			if( SystemInfo.isLinux )
+				unsupported( windowDecorationsCheckBoxMenuItem );
+			else
+				windowDecorationsCheckBoxMenuItem.setSelected( FlatLaf.isUseNativeWindowDecorations() );
 			menuBarEmbeddedCheckBoxMenuItem.setSelected( UIManager.getBoolean( "TitlePane.menuBarEmbedded" ) );
 			unifiedTitleBarMenuItem.setSelected( UIManager.getBoolean( "TitlePane.unifiedBackground" ) );
 			showTitleBarIconMenuItem.setSelected( UIManager.getBoolean( "TitlePane.showIcon" ) );

flatlaf-demo/src/main/java/com/formdev/flatlaf/demo/FlatLafDemo.java

@@ -17,6 +17,8 @@
 package com.formdev.flatlaf.demo;
 
 import java.awt.Dimension;
+import javax.swing.JDialog;
+import javax.swing.JFrame;
 import javax.swing.SwingUtilities;
 import com.formdev.flatlaf.FlatLaf;
 import com.formdev.flatlaf.extras.FlatInspector;
@@ -52,6 +54,13 @@ public class FlatLafDemo
 			System.setProperty( "apple.awt.application.appearance", "system" );
 		}
 
+		// Linux
+		if( SystemInfo.isLinux ) {
+			// enable custom window decorations
+			JFrame.setDefaultLookAndFeelDecorated( true );
+			JDialog.setDefaultLookAndFeelDecorated( true );
+		}
+
 		if( FlatLafDemo.screenshotsMode && !SystemInfo.isJava_9_orLater && System.getProperty( "flatlaf.uiScale" ) == null )
 			System.setProperty( "flatlaf.uiScale", "2x" );