ui: readd window opacity setting

The undecorated frame requirement appears to not be real, at least on Windows.

AWT applies translucency via setting WS_EX_LAYERED to make the window layered and then changes the opacity of the layered window with SetLayeredWindowAttributes.

This appears to work regardless of the frame state, even with custom chrome off.

runelite-client/src/main/java/net/runelite/client/config/RuneLiteConfig.java

@@ -133,6 +133,22 @@ default boolean enableCustomChrome()
 		return OSType.getOSType() == OSType.Windows;
 	}
 
+	@Range(
+		min = 10,
+		max = 100
+	)
+	@ConfigItem(
+		keyName = "uiWindowOpacity",
+		name = "Window opacity",
+		description = "Set the windows opacity.",
+		position = 16,
+		section = windowSettings
+	)
+	default int windowOpacity()
+	{
+		return 100;
+	}
+
 	@ConfigItem(
 		keyName = "gameAlwaysOnTop",
 		name = "Always on top",

runelite-client/src/main/java/net/runelite/client/ui/ClientUI.java

@@ -1114,6 +1114,11 @@ private void updateFrameConfig(boolean updateResizable)
 			return;
 		}
 
+		if (frame.getGraphicsConfiguration().getDevice().getFullScreenWindow() == null)
+		{
+			frame.setOpacity(((float) config.windowOpacity()) / 100.0f);
+		}
+
 		if (config.usernameInTitle() && (client instanceof Client))
 		{
 			final Player player = ((Client) client).getLocalPlayer();

runelite-client/src/main/java/net/runelite/client/ui/ContainableFrame.java

@@ -27,6 +27,7 @@
 import com.formdev.flatlaf.ui.FlatNativeWindowsLibrary;
 import java.awt.Dimension;
 import java.awt.Frame;
+import java.awt.IllegalComponentStateException;
 import java.awt.Insets;
 import java.awt.Rectangle;
 import java.awt.geom.AffineTransform;
@@ -52,6 +53,7 @@ public enum Mode
 	private boolean rightSideSuction;
 	private boolean boundsOpSet;
 	private boolean scaleMinSize = false;
+	private boolean overrideUndecorated;
 
 	@Override
 	@SuppressWarnings("deprecation")
@@ -259,4 +261,30 @@ void updateContainsInScreen()
 			FlatNativeWindowsLibrary.setContainInScreen(this, containedInScreen == Mode.ALWAYS);
 		}
 	}
+
+	@Override
+	public void setOpacity(float opacity)
+	{
+		// JDK-6993784 requires the frame to be undecorated to apply opacity, but in practice it works on Windows regardless.
+		// Temporarily pretend to be an undecorated frame to satisfy Frame.setOpacity().
+		overrideUndecorated = true;
+		try
+		{
+			super.setOpacity(opacity);
+		}
+		catch (IllegalComponentStateException | UnsupportedOperationException | IllegalArgumentException ex)
+		{
+			log.warn("unable to set opacity {}", opacity, ex);
+		}
+		finally
+		{
+			overrideUndecorated = false;
+		}
+	}
+
+	@Override
+	public boolean isUndecorated()
+	{
+		return overrideUndecorated || super.isUndecorated();
+	}
 }