Native window decorations: when window is initially shown, fill background with window background color (instead of white), which avoids flickering in dark themes (issue #339)

CHANGELOG.md

@@ -11,9 +11,14 @@ FlatLaf Change Log
   maximized windows. (issue #358)
 - Native window decorations (Windows 10 only):
   - Fixed occasional application crash in `flatlaf-windows.dll`. (issue #357)
-  - When resizing a window to the right or to the bottom, then first fill the
-    new space with the window background color (instead of black) before the
-    layout is updated.
+  - When window is initially shown, fill background with window background color
+    (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.
+  - When resizing a window at the left/top edge, then first fill the new space
+    with the window background color (instead of garbage) before the layout is
+    updated.
 
 
 ## 1.4

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

@@ -60,7 +60,6 @@ import com.sun.jna.platform.win32.WinDef.LRESULT;
 import com.sun.jna.platform.win32.WinDef.RECT;
 import com.sun.jna.platform.win32.WinDef.UINT_PTR;
 import com.sun.jna.platform.win32.WinDef.WPARAM;
-import com.sun.jna.platform.win32.WinNT.HANDLE;
 import com.sun.jna.platform.win32.WinUser.HMONITOR;
 import com.sun.jna.platform.win32.WinUser.WindowProc;
 import com.sun.jna.win32.W32APIOptions;
@@ -291,6 +290,7 @@ public class FlatWindowsNativeWindowBorder
 		private static final int GWLP_WNDPROC = -4;
 
 		private static final int
+			WM_ERASEBKGND = 0x0014,
 			WM_NCCALCSIZE = 0x0083,
 			WM_NCHITTEST = 0x0084,
 			WM_NCRBUTTONUP = 0x00A5,
@@ -330,6 +330,7 @@ public class FlatWindowsNativeWindowBorder
 		private final HWND hwnd;
 		private final BaseTSD.LONG_PTR defaultWndProc;
 		private int wmSizeWParam = -1;
+		private HBRUSH background;
 
 		private int titleBarHeight;
 		private Rectangle[] hitTestSpots;
@@ -368,6 +369,8 @@ public class FlatWindowsNativeWindowBorder
 			updateFrame( 0 );
 
 			// cleanup
+			if( background != null )
+				GDI32.INSTANCE.DeleteObject( background );
 			window = null;
 		}
 
@@ -405,13 +408,11 @@ public class FlatWindowsNativeWindowBorder
 
 		private void setWindowBackground( int r, int g, int b ) {
 			// delete old background brush
-			ULONG_PTR oldBrush = User32.INSTANCE.GetClassLongPtr( hwnd, GCLP_HBRBACKGROUND );
-			if( oldBrush != null && oldBrush.longValue() != 0 )
-				GDI32.INSTANCE.DeleteObject( new HANDLE( oldBrush.toPointer() ) );
+			if( background != null )
+				GDI32.INSTANCE.DeleteObject( background );
 
 			// create new background brush
-			HBRUSH brush = GDI32Ex.INSTANCE.CreateSolidBrush( RGB( r, g, b ) );
-			User32Ex.INSTANCE.SetClassLongPtr( hwnd, GCLP_HBRBACKGROUND, brush );
+			background = GDI32Ex.INSTANCE.CreateSolidBrush( RGB( r, g, b ) );
 		}
 
 		/**
@@ -440,6 +441,9 @@ public class FlatWindowsNativeWindowBorder
 						wParam = new WPARAM( wmSizeWParam );
 					break;
 
+				case WM_ERASEBKGND:
+					return WmEraseBkgnd( hwnd, uMsg, wParam, lParam );
+
 				case WM_DESTROY:
 					return WmDestroy( hwnd, uMsg, wParam, lParam );
 			}
@@ -464,11 +468,30 @@ public class FlatWindowsNativeWindowBorder
 
 			// cleanup
 			windowsMap.remove( window );
+			if( background != null )
+				GDI32.INSTANCE.DeleteObject( background );
 			window = null;
 
 			return lResult;
 		}
 
+		/**
+		 * Handle WM_ERASEBKGND
+		 *
+		 * https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-erasebkgnd
+		 */
+		LRESULT WmEraseBkgnd( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam ) {
+			if( background == null )
+				return new LRESULT( 0 );
+
+			// fill background
+			HDC hdc = new HDC( wParam.toPointer() );
+		    RECT rect = new RECT();
+		    User32.INSTANCE.GetClientRect( hwnd, rect );
+		    User32Ex.INSTANCE.FillRect( hdc, rect, background );
+		    return new LRESULT( 1 );
+		}
+
 		/**
 		 * Handle WM_NCCALCSIZE
 		 *
@@ -729,7 +752,7 @@ public class FlatWindowsNativeWindowBorder
 		LONG_PTR SetWindowLong( HWND hWnd, int nIndex, LONG_PTR wndProc );
 		LRESULT CallWindowProc( LONG_PTR lpPrevWndFunc, HWND hWnd, int uMsg, WPARAM wParam, LPARAM lParam );
 
-		LONG_PTR SetClassLongPtr( HWND hWnd, int nIndex, HANDLE wndProc );
+		int FillRect( HDC hDC, RECT lprc, HBRUSH hbr );
 
 		int GetDpiForWindow( HWND hwnd );
 		int GetSystemMetricsForDpi( int nIndex, int dpi );

flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.cpp

@@ -87,6 +87,7 @@ FlatWndProc::FlatWndProc() {
 	hwnd = NULL;
 	defaultWndProc = NULL;
 	wmSizeWParam = -1;
+	background = NULL;
 }
 
 HWND FlatWndProc::install( JNIEnv *env, jobject obj, jobject window ) {
@@ -135,6 +136,8 @@ void FlatWndProc::uninstall( JNIEnv *env, jobject obj, HWND hwnd ) {
 
 	// cleanup
 	env->DeleteGlobalRef( fwp->obj );
+	if( fwp->background != NULL )
+		::DeleteObject( fwp->background );
 	delete fwp;
 }
 
@@ -182,14 +185,16 @@ void FlatWndProc::updateFrame( HWND hwnd, int state ) {
 }
 
 void FlatWndProc::setWindowBackground( HWND hwnd, int r, int g, int b ) {
+	FlatWndProc* fwp = (FlatWndProc*) hwndMap->get( hwnd );
+	if( fwp == NULL )
+		return;
+
 	// delete old background brush
-	HBRUSH oldBrush = (HBRUSH) ::GetClassLongPtr( hwnd, GCLP_HBRBACKGROUND );
-	if( oldBrush != NULL )
-		::DeleteObject( oldBrush );
+	if( fwp->background != NULL )
+		::DeleteObject( fwp->background );
 
 	// create new background brush
-	HBRUSH brush = ::CreateSolidBrush( RGB( r, g, b ) );
-	::SetClassLongPtr( hwnd, GCLP_HBRBACKGROUND, (LONG_PTR) brush );
+	fwp->background = ::CreateSolidBrush( RGB( r, g, b ) );
 }
 
 LRESULT CALLBACK FlatWndProc::StaticWindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam ) {
@@ -224,12 +229,16 @@ LRESULT CALLBACK FlatWndProc::WindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, L
 				wParam = wmSizeWParam;
 			break;
 
+		case WM_ERASEBKGND:
+			return WmEraseBkgnd( hwnd, uMsg, wParam, lParam );
+
 		case WM_DESTROY:
 			return WmDestroy( hwnd, uMsg, wParam, lParam );
 	}
 
 	return ::CallWindowProc( defaultWndProc, hwnd, uMsg, wParam, lParam );
 }
+
 /**
  * Handle WM_DESTROY
  *
@@ -243,6 +252,8 @@ LRESULT FlatWndProc::WmDestroy( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lPara
 
 	// cleanup
 	getEnv()->DeleteGlobalRef( obj );
+	if( background != NULL )
+		::DeleteObject( background );
 	hwndMap->remove( hwnd );
 	delete this;
 
@@ -250,6 +261,23 @@ LRESULT FlatWndProc::WmDestroy( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lPara
 	return ::CallWindowProc( defaultWndProc2, hwnd, uMsg, wParam, lParam );
 }
 
+/**
+ * Handle WM_ERASEBKGND
+ *
+ * https://docs.microsoft.com/en-us/windows/win32/winmsg/wm-erasebkgnd
+ */
+LRESULT FlatWndProc::WmEraseBkgnd( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam ) {
+	if( background == NULL )
+		return FALSE;
+
+	// fill background
+	HDC hdc = (HDC) wParam;
+	RECT rect;
+	::GetClientRect( hwnd, &rect );
+	::FillRect( hdc, &rect, background );
+	return TRUE;
+}
+
 /**
  * Handle WM_NCCALCSIZE
  *

flatlaf-natives/flatlaf-natives-windows/src/main/cpp/FlatWndProc.h

@@ -42,6 +42,7 @@ private:
 	HWND hwnd;
 	WNDPROC defaultWndProc;
 	int wmSizeWParam;
+	HBRUSH background;
 
 	FlatWndProc();
 	static void initIDs( JNIEnv *env, jobject obj );
@@ -49,6 +50,7 @@ private:
 	static LRESULT CALLBACK StaticWindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam );
 	LRESULT CALLBACK WindowProc( HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam );
 	LRESULT WmDestroy( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam );
+	LRESULT WmEraseBkgnd( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam );
 	LRESULT WmNcCalcSize( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam );
 	LRESULT WmNcHitTest( HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam );