From 24c2b3cf01885a5d60ff49a8bcd89f889f093456 Mon Sep 17 00:00:00 2001
From: Jens Weggemann <jweggemann@nvidia.com>
Date: Wed, 21 Jan 2026 19:52:47 +0100
Subject: [PATCH] Win32: Clear to black in WM_PAINT for transparent framebuffer
 windows

The DWM redirection surface may contain undefined alpha values,
causing the window to composite incorrectly. PatBlt with BLACKNESS
clears both RGB and alpha to zero.
---
 src/win32_window.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/src/win32_window.c b/src/win32_window.c
index 6427a673..2af2f05b 100644
--- a/src/win32_window.c
+++ b/src/win32_window.c
@@ -1139,6 +1139,22 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM l
         case WM_PAINT:
         {
             _glfwInputWindowDamage(window);
+
+            // The DWM redirection surface starts with undefined alpha.
+            // Clear to BLACKNESS which should zero out the alpha components as well.
+            if (window->win32.transparent)
+            {
+                PAINTSTRUCT ps;
+                HDC hdc = BeginPaint(hWnd, &ps);
+                PatBlt(hdc,
+                       ps.rcPaint.left,
+                       ps.rcPaint.top,
+                       ps.rcPaint.right  - ps.rcPaint.left,
+                       ps.rcPaint.bottom - ps.rcPaint.top,
+                       BLACKNESS);
+                EndPaint(hWnd, &ps);
+                return 0;
+            }
             break;
         }