Backends: Win32: Viewports: fixed an issue when closing a window from the OS close button (with io.ConfigViewportsNoDecoration=false) while user code is discarding the 'bool *p_open=false output' from Begin()

backends/imgui_impl_win32.cpp

@@ -23,6 +23,7 @@
 // CHANGELOG
 // (minor and older changes stripped away, please see git history for details)
 //  2025-XX-XX: Platform: Added support for multiple windows via the ImGuiPlatformIO interface.
+//  2025-03-26: [Docking] Viewports: fixed an issue when closing a window from the OS close button (with io.ConfigViewportsNoDecoration = false) while user code was discarding the 'bool* p_open = false' output from Begin(). Because we allowed the Win32 window to close early, Windows destroyed it and our imgui window became not visible even though user code was still submitting it.
 //  2025-03-10: When dealing with OEM keys, use scancodes instead of translated keycodes to choose ImGuiKey values. (#7136, #7201, #7206, #7306, #7670, #7672, #8468)
 //  2025-02-21: [Docking] WM_SETTINGCHANGE's SPI_SETWORKAREA message also triggers a refresh of monitor list. (#8415)
 //  2025-02-18: Added ImGuiMouseCursor_Wait and ImGuiMouseCursor_Progress mouse cursor support.
@@ -1357,14 +1358,14 @@ static LRESULT CALLBACK ImGui_ImplWin32_WndProcHandler_PlatformWindow(HWND hWnd,
     ImGuiPlatformIO& platform_io = ImGui::GetPlatformIO(ctx);
     LRESULT result = 0;
     if (ImGui_ImplWin32_WndProcHandlerEx(hWnd, msg, wParam, lParam, io))
-        result = true;
+        result = 1;
     else if (ImGuiViewport* viewport = ImGui_ImplWin32_FindViewportByPlatformHandle(platform_io, hWnd))
     {
         switch (msg)
         {
         case WM_CLOSE:
             viewport->PlatformRequestClose = true;
-            break;
+            return 0; // 0 = Operating system will ignore the message and not destroy the window. We close ourselves.
         case WM_MOVE:
             viewport->PlatformRequestMove = true;
             break;

docs/CHANGELOG.txt

@@ -48,6 +48,15 @@ Other changes:
 - Style, InputText: added ImGuiCol_InputTextCursor to configure color of
   the InputText cursor/caret. (#7031)
 
+Docking+Viewports Branch:
+
+- Backends: Win32: Viewports: fixed an issue when closing a window from 
+  the OS close button (with io.ConfigViewportsNoDecoration=false) while 
+  user code is discarding the 'bool *p_open=false output' from Begin(). 
+  Because we allowed the Win32 window to close early, Windows destroyed
+  it and our imgui window became not visible even though user code was
+  still submitting it.
+
 
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  VERSION 1.91.9b (Released 2025-03-17)