From 2e222bcdeafcddd7bb23d417ead016a34c25af4d Mon Sep 17 00:00:00 2001
From: Karl Tauber <karl@jformdesigner.com>
Date: Fri, 18 Feb 2022 22:24:36 +0100
Subject: [PATCH] Native window decorations (Windows 10/11 only): fixed
 rendering artifacts on HiDPI screens when dragging window partly offscreen
 and back into screen bounds (issue #477)

---
 CHANGELOG.md                                  |  3 +++
 .../FlatWindowsNativeWindowBorder.java        | 27 +++++++++++++++++++
 .../src/main/cpp/FlatWndProc.cpp              | 22 +++++++++++++++
 .../src/main/cpp/FlatWndProc.h                |  2 ++
 4 files changed, 54 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d94ecd82..eb3d1d07 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,9 @@ FlatLaf Change Log
 
 ## 2.1-SNAPSHOT
 
+- Native window decorations (Windows 10/11 only): Fixed rendering artifacts on
+  HiDPI screens when dragging window partly offscreen and back into screen
+  bounds. (issue #477)
 - Repaint component when setting client property `JComponent.outline` (issue
   #480).
 
diff --git a/flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java b/flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java
index 5bc8bf45..ba23a195 100644
--- a/flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java
+++ b/flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java
@@ -288,6 +288,7 @@ public class FlatWindowsNativeWindowBorder
 		private static final int GWLP_WNDPROC = -4;
 
 		private static final int
+			WM_MOVE = 0x0003,
 			WM_ERASEBKGND = 0x0014,
 			WM_NCCALCSIZE = 0x0083,
 			WM_NCHITTEST = 0x0084,
@@ -301,6 +302,10 @@ public class FlatWindowsNativeWindowBorder
 			WM_LBUTTONDOWN = 0x0201,
 			WM_LBUTTONUP = 0x0202,
 
+			WM_MOVING = 0x0216,
+			WM_ENTERSIZEMOVE = 0x0231,
+			WM_EXITSIZEMOVE = 0x0232,
+
 			WM_DWMCOLORIZATIONCOLORCHANGED = 0x0320;
 
 		// WM_SIZE wParam
@@ -341,6 +346,8 @@ public class FlatWindowsNativeWindowBorder
 		private final LONG_PTR defaultWndProc;
 		private int wmSizeWParam = -1;
 		private HBRUSH background;
+		private boolean isMovingOrSizing;
+		private boolean isMoving;
 
 		// Swing coordinates/values may be scaled on a HiDPI screen
 		private int titleBarHeight;
@@ -477,7 +484,27 @@ public class FlatWindowsNativeWindowBorder
 						wParam = new WPARAM( wmSizeWParam );
 					break;
 
+				case WM_ENTERSIZEMOVE:
+					isMovingOrSizing = true;
+					break;
+
+				case WM_EXITSIZEMOVE:
+					isMovingOrSizing = isMoving = false;
+					break;
+
+				case WM_MOVE:
+				case WM_MOVING:
+					if( isMovingOrSizing )
+						isMoving = true;
+					break;
+
 				case WM_ERASEBKGND:
+					// do not erase background while the user is moving the window,
+					// otherwise there may be rendering artifacts on HiDPI screens with Java 9+
+					// when dragging the window partly offscreen and back into the screen bounds
+					if( isMoving )
+						return new LRESULT( 0 );
+
 					return WmEraseBkgnd( hwnd, uMsg, wParam, lParam );
 
 				case WM_DESTROY:
diff --git a/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.cpp b/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.cpp
index 38d4550d..b2f72ce1 100644
--- a/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.cpp
+++ b/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.cpp
@@ -88,6 +88,8 @@ FlatWndProc::FlatWndProc() {
 	defaultWndProc = NULL;
 	wmSizeWParam = -1;
 	background = NULL;
+	isMovingOrSizing = false;
+	isMoving = false;
 }
 
 HWND FlatWndProc::install( JNIEnv *env, jobject obj, jobject window ) {
@@ -250,7 +252,27 @@ LRESULT CALLBACK FlatWndProc::WindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, L
 				wParam = wmSizeWParam;
 			break;
 
+		case WM_ENTERSIZEMOVE:
+			isMovingOrSizing = true;
+			break;
+
+		case WM_EXITSIZEMOVE:
+			isMovingOrSizing = isMoving = false;
+			break;
+
+		case WM_MOVE:
+		case WM_MOVING:
+			if( isMovingOrSizing )
+				isMoving = true;
+			break;
+
 		case WM_ERASEBKGND:
+			// do not erase background while the user is moving the window,
+			// otherwise there may be rendering artifacts on HiDPI screens with Java 9+
+			// when dragging the window partly offscreen and back into the screen bounds
+			if( isMoving )
+				return FALSE;
+
 			return WmEraseBkgnd( hwnd, uMsg, wParam, lParam );
 
 		case WM_DESTROY:
diff --git a/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.h b/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.h
index 646f5a57..768b7779 100644
--- a/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.h
+++ b/flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.h
@@ -43,6 +43,8 @@ private:
 	WNDPROC defaultWndProc;
 	int wmSizeWParam;
 	HBRUSH background;
+	bool isMovingOrSizing;
+	bool isMoving;
 
 	FlatWndProc();
 	static void initIDs( JNIEnv *env, jobject obj );