Window decorations (Windows 10/11 only):

- improved diagonal window resizing on top-left and top-right window corners
- top window resize area now also covers iconify/maximize/close buttons
(issue #1015)

CHANGELOG.md

@@ -4,6 +4,9 @@ FlatLaf Change Log
 ## 3.6.1-SNAPSHOT
 
 - Extras: Support JSVG 2.0.0. Minimum JSVG version is now 1.6.0. (issue #997)
+- FlatLaf window decorations (Windows 10/11 only): Improved diagonal window
+  resizing on top-left and top-right window corners. Top window resize area now
+  also covers iconify/maximize/close buttons. (issue #1015)
 - ToggleButton: Styling `selectedForeground` did not work if `foreground` is
   also styled. (issue #1017)
 - JideSplitButton: Fixed updating popup when switching theme. (issue #1000)

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

@@ -21,6 +21,7 @@ import java.awt.Dialog;
 import java.awt.EventQueue;
 import java.awt.Frame;
 import java.awt.GraphicsConfiguration;
+import java.awt.Insets;
 import java.awt.Point;
 import java.awt.Rectangle;
 import java.awt.Window;
@@ -39,6 +40,7 @@ import javax.swing.event.ChangeListener;
 import javax.swing.event.EventListenerList;
 import com.formdev.flatlaf.util.LoggingFacade;
 import com.formdev.flatlaf.util.SystemInfo;
+import com.formdev.flatlaf.util.UIScale;
 
 //
 // Interesting resources:
@@ -282,6 +284,8 @@ class FlatWindowsNativeWindowBorder
 			HTMINBUTTON = 8,
 			HTMAXBUTTON = 9,
 			HTTOP = 12,
+			HTTOPLEFT = 13,
+			HTTOPRIGHT = 14,
 			HTCLOSE = 20;
 
 		private Window window;
@@ -341,6 +345,31 @@ class FlatWindowsNativeWindowBorder
 			// scale-down mouse x/y because Swing coordinates/values may be scaled on a HiDPI screen
 			Point pt = scaleDown( x, y );
 
+			// limit top resize border to 4px, which seems to be standard for modern WinUI apps
+			if( isOnResizeBorder && pt.y > UIScale.scale( 4 ) )
+				isOnResizeBorder = false;
+
+			if( isOnResizeBorder ) {
+				Insets insets = window.getInsets();
+
+				// return HTTOPLEFT if mouse is over top resize border on left side
+				//   - hovering mouse shows top-left resize cursor
+				//   - left-click-and-drag resizes window
+				if( pt.x <= insets.left + UIScale.scale( 12 ) )
+					return HTTOPLEFT;
+
+				// return HTTOPRIGHT if mouse is over top resize border on right side
+				//   - hovering mouse shows top-right resize cursor
+				//   - left-click-and-drag resizes window
+				if( pt.x >= window.getWidth() - insets.right - UIScale.scale( 12 ) )
+					return HTTOPRIGHT;
+
+				// return HTTOP if mouse is over top resize border
+				//   - hovering mouse shows vertical resize cursor
+				//   - left-click-and-drag vertically resizes window
+				return HTTOP;
+			}
+
 			// return HTSYSMENU if mouse is over application icon
 			//   - left-click on HTSYSMENU area shows system menu
 			//   - double-left-click sends WM_CLOSE
@@ -364,12 +393,6 @@ class FlatWindowsNativeWindowBorder
 			if( contains( closeButtonBounds, pt ) )
 				return HTCLOSE;
 
-			// return HTTOP if mouse is over top resize border
-			//   - hovering mouse shows vertical resize cursor
-			//   - left-click and drag vertically resizes window
-			if( isOnResizeBorder )
-				return HTTOP;
-
 			boolean isOnTitleBar = (pt.y < titleBarHeight);
 			if( isOnTitleBar ) {
 				// return HTCLIENT if mouse is over any Swing component in title bar

flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java

@@ -24,6 +24,7 @@ import java.awt.Dialog;
 import java.awt.EventQueue;
 import java.awt.Frame;
 import java.awt.GraphicsConfiguration;
+import java.awt.Insets;
 import java.awt.Point;
 import java.awt.Rectangle;
 import java.awt.Window;
@@ -42,6 +43,7 @@ import javax.swing.event.ChangeListener;
 import javax.swing.event.EventListenerList;
 import com.formdev.flatlaf.ui.FlatNativeWindowBorder;
 import com.formdev.flatlaf.util.SystemInfo;
+import com.formdev.flatlaf.util.UIScale;
 import com.sun.jna.Native;
 import com.sun.jna.Pointer;
 import com.sun.jna.Structure;
@@ -326,6 +328,8 @@ public class FlatWindowsNativeWindowBorder
 			HTMINBUTTON = 8,
 			HTMAXBUTTON = 9,
 			HTTOP = 12,
+			HTTOPLEFT = 13,
+			HTTOPRIGHT = 14,
 			HTCLOSE = 20;
 
 		private static final int ABS_AUTOHIDE = 0x0000001;
@@ -674,6 +678,35 @@ public class FlatWindowsNativeWindowBorder
 			// scale-down mouse x/y because Swing coordinates/values may be scaled on a HiDPI screen
 			Point pt = scaleDown( x, y );
 
+			int resizeBorderHeight = getResizeHandleHeight();
+			boolean isOnResizeBorder = (y < resizeBorderHeight) &&
+				(User32.INSTANCE.GetWindowLong( hwnd, GWL_STYLE ) & WS_THICKFRAME) != 0;
+
+			// limit top resize border to 4px, which seems to be standard for modern WinUI apps
+			if( isOnResizeBorder && pt.y > UIScale.scale( 4 ) )
+				isOnResizeBorder = false;
+
+			if( isOnResizeBorder ) {
+				Insets insets = window.getInsets();
+
+				// return HTTOPLEFT if mouse is over top resize border on left side
+				//   - hovering mouse shows top-left resize cursor
+				//   - left-click-and-drag resizes window
+				if( pt.x <= insets.left + UIScale.scale( 12 ) )
+					return new LRESULT( HTTOPLEFT );
+
+				// return HTTOPRIGHT if mouse is over top resize border on right side
+				//   - hovering mouse shows top-right resize cursor
+				//   - left-click-and-drag resizes window
+				if( pt.x >= window.getWidth() - insets.right - UIScale.scale( 12 ) )
+					return new LRESULT( HTTOPRIGHT );
+
+				// return HTTOP if mouse is over top resize border
+				//   - hovering mouse shows vertical resize cursor
+				//   - left-click-and-drag vertically resizes window
+				return new LRESULT( HTTOP );
+			}
+
 			// return HTSYSMENU if mouse is over application icon
 			//   - left-click on HTSYSMENU area shows system menu
 			//   - double-left-click sends WM_CLOSE
@@ -697,16 +730,6 @@ public class FlatWindowsNativeWindowBorder
 			if( contains( closeButtonBounds, pt ) )
 				return new LRESULT( HTCLOSE );
 
-			int resizeBorderHeight = getResizeHandleHeight();
-			boolean isOnResizeBorder = (y < resizeBorderHeight) &&
-				(User32.INSTANCE.GetWindowLong( hwnd, GWL_STYLE ) & WS_THICKFRAME) != 0;
-
-			// return HTTOP if mouse is over top resize border
-			//   - hovering mouse shows vertical resize cursor
-			//   - left-click and drag vertically resizes window
-			if( isOnResizeBorder )
-				return new LRESULT( HTTOP );
-
 			boolean isOnTitleBar = (pt.y < titleBarHeight);
 			if( isOnTitleBar ) {
 				// return HTCLIENT if mouse is over any Swing component in title bar