Panel: rounded background of panel with rounded corners is now painte…

…d even if panel is not opaque (issue #840)

CHANGELOG.md

@@ -9,6 +9,8 @@ FlatLaf Change Log
   indeterminate progress bar UI or using JProgressBar.setIndeterminate(false)
   not on AWT thread, because this may throw NPE in FlatProgressBarUI.paint().
   (issues #841 and #830)
+- Panel: Rounded background of panel with rounded corners is now painted even if
+  panel is not opaque. (issue #840)
 
 
 ## 3.4.1
@@ -20,7 +22,7 @@ FlatLaf Change Log
 - TabbedPane: Fixed swapped back and forward scroll buttons when using
   `TabbedPane.scrollButtonsPlacement = trailing` (regression in FlatLaf 3.3).
 - Fixed missing window top border on Windows 10 in "full window content" mode.
-  (issue 809)
+  (issue #809)
 - Extras:
   - `FlatSVGIcon` color filters now support linear gradients. (PR #817)
   - `FlatSVGIcon`: Use log level `CONFIG` instead of `SEVERE` and allow
@@ -65,7 +67,7 @@ FlatLaf Change Log
 - Fonts: Updated **Inter** to
   [v4.0](https://github.com/rsms/inter/releases/tag/v4.0).
 - Table: Select all text in cell editor when starting editing using `F2` key.
-  (issue 652)
+  (issue #652)
 
 #### Fixed bugs
 
@@ -93,7 +95,7 @@ FlatLaf Change Log
 #### Fixed bugs
 
 - Button and ToggleButton: Selected buttons did not use explicitly set
-  foreground color. (issue 756)
+  foreground color. (issue #756)
 - FileChooser: Catch NPE in Java 21 when getting icon for `.exe` files that use
   default Windows exe icon. (see
   [JDK-8320692](https://bugs.openjdk.org/browse/JDK-8320692))
@@ -846,7 +848,7 @@ FlatLaf Change Log
 - Native window decorations (Windows 10 only):
   - Fixed occasional application crash in `flatlaf-windows.dll`. (issue #357)
   - When window is initially shown, fill background with window background color
-    (instead of white), which avoids flickering in dark themes. (issue 339)
+    (instead of white), which avoids flickering in dark themes. (issue #339)
   - When resizing a window at the right/bottom edge, then first fill the new
     space with the window background color (instead of black) before the layout
     is updated.

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

@@ -160,29 +160,30 @@ public Object getStyleableValue( JComponent c, String key ) {
 
 	@Override
 	public void update( Graphics g, JComponent c ) {
+		int arc = (this.arc >= 0)
+			? this.arc
+			: ((c.getBorder() instanceof FlatLineBorder)
+				? ((FlatLineBorder)c.getBorder()).getArc()
+				: 0);
+
 		// fill background
 		if( c.isOpaque() ) {
-			int width = c.getWidth();
-			int height = c.getHeight();
-			int arc = (this.arc >= 0)
-				? this.arc
-				: ((c.getBorder() instanceof FlatLineBorder)
-					? ((FlatLineBorder)c.getBorder()).getArc()
-					: 0);
-
-			// fill background with parent color to avoid garbage in rounded corners
-			if( arc > 0 )
+			if( arc > 0 ) {
+				// fill background with parent color to avoid garbage in rounded corners
 				FlatUIUtils.paintParentBackground( g, c );
+			} else {
+				g.setColor( c.getBackground() );
+				g.fillRect( 0, 0, c.getWidth(), c.getHeight() );
+			}
+		}
 
+		// fill rounded rectangle if having rounded corners
+		if( arc > 0 ) {
 			g.setColor( c.getBackground() );
-			if( arc > 0 ) {
-				// fill rounded rectangle if having rounded corners
-				Object[] oldRenderingHints = FlatUIUtils.setRenderingHints( g );
-				FlatUIUtils.paintComponentBackground( (Graphics2D) g, 0, 0, width, height,
-					0, UIScale.scale( arc ) );
-				FlatUIUtils.resetRenderingHints( g, oldRenderingHints );
-			} else
-				g.fillRect( 0, 0, width, height );
+			Object[] oldRenderingHints = FlatUIUtils.setRenderingHints( g );
+			FlatUIUtils.paintComponentBackground( (Graphics2D) g, 0, 0, c.getWidth(), c.getHeight(),
+				0, UIScale.scale( arc ) );
+			FlatUIUtils.resetRenderingHints( g, oldRenderingHints );
 		}
 
 		paint( g, c );