Backends: Win32: Viewports: handle WM_DPICHANGED in backend when ImGuiConfigFlags_DpiEnableScaleViewports flag is enabled.

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.
+//  2025-06-02: [Docking] WM_DPICHANGED also apply ImGuiConfigFlags_DpiEnableScaleViewports for main viewport instead of letting it be done by application code.
 //  2025-04-30: Inputs: Fixed an issue where externally losing mouse capture (due to e.g. focus loss) would fail to claim it again the next subsequent click. (#8594)
 //  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)
@@ -705,6 +706,10 @@ static ImGuiMouseSource ImGui_ImplWin32_GetMouseSourceFromMessageExtraInfo()
 extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);                // Use ImGui::GetCurrentContext()
 extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandlerEx(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam, ImGuiIO& io); // Doesn't use ImGui::GetCurrentContext()
 
+#ifndef WM_DPICHANGED
+#define WM_DPICHANGED 0x02E0 // From Windows SDK 8.1+ headers
+#endif
+
 IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
 {
     // Most backends don't have silent checks like this one, but we need it because WndProc are called early in CreateWindow().
@@ -899,6 +904,13 @@ IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandlerEx(HWND hwnd, UINT msg, WPA
         if (wParam == SPI_SETWORKAREA)
             bd->WantUpdateMonitors = true;
         return 0;
+    case WM_DPICHANGED:
+    {
+        const RECT* suggested_rect = (RECT*)lParam;
+        if (io.ConfigFlags & ImGuiConfigFlags_DpiEnableScaleViewports)
+            ::SetWindowPos(hwnd, nullptr, suggested_rect->left, suggested_rect->top, suggested_rect->right - suggested_rect->left, suggested_rect->bottom - suggested_rect->top, SWP_NOZORDER | SWP_NOACTIVATE);
+        return 0;
+    }
     }
     return 0;
 }

docs/CHANGELOG.txt

@@ -171,14 +171,16 @@ Docking+Viewports Branch:
   (this per-se doesn't fix the font quality which requires setting RasterizerDensity
   separately. More on this later as it should soon become automatic).
   (#1065, #1542, #1676, #1786, #2826, #3757, #5081, #5580, #5592, #6465, #7273, #7779 etc.)
-- Viewports: fixed handling of simultaneous move + resize (e.g. toggling maximized) when
-  ImGuiConfigFlags_DpiEnableScaleViewports is enabled.
+- Viewports: fixed handling of simultaneous move + resize (e.g. toggling maximized)
+  when ImGuiConfigFlags_DpiEnableScaleViewports is enabled.
 - 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.
+- Backends: Win32: Viewports: handle WM_DPICHANGED in backend when
+  ImGuiConfigFlags_DpiEnableScaleViewports is enabled.
 - Backends: GLFW, SDL2, SDL3, Apple: provide Platform_GetWindowFramebufferScale handler,
   (#1065, #1542, #1676, #1786, #2826, #3757, #5081, #5580, #5592, #6465, #7273, #7779 etc.)
 - Backends: SDLGPU3 for SDL3: added multi-viewport support. (#8573) [@Lekoopapaul]

examples/example_win32_directx11/main.cpp

@@ -264,10 +264,6 @@ void CleanupRenderTarget()
     if (g_mainRenderTargetView) { g_mainRenderTargetView->Release(); g_mainRenderTargetView = nullptr; }
 }
 
-#ifndef WM_DPICHANGED
-#define WM_DPICHANGED 0x02E0 // From Windows SDK 8.1+ headers
-#endif
-
 // Forward declare message handler from imgui_impl_win32.cpp
 extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
 
@@ -296,15 +292,6 @@ LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
     case WM_DESTROY:
         ::PostQuitMessage(0);
         return 0;
-    case WM_DPICHANGED:
-        if (ImGui::GetIO().ConfigFlags & ImGuiConfigFlags_DpiEnableScaleViewports)
-        {
-            //const int dpi = HIWORD(wParam);
-            //printf("WM_DPICHANGED to %d (%.0f%%)\n", dpi, (float)dpi / 96.0f * 100.0f);
-            const RECT* suggested_rect = (RECT*)lParam;
-            ::SetWindowPos(hWnd, nullptr, suggested_rect->left, suggested_rect->top, suggested_rect->right - suggested_rect->left, suggested_rect->bottom - suggested_rect->top, SWP_NOZORDER | SWP_NOACTIVATE);
-        }
-        break;
     }
     return ::DefWindowProcW(hWnd, msg, wParam, lParam);
 }

examples/example_win32_directx9/main.cpp

@@ -250,10 +250,6 @@ void ResetDevice()
     ImGui_ImplDX9_CreateDeviceObjects();
 }
 
-#ifndef WM_DPICHANGED
-#define WM_DPICHANGED 0x02E0 // From Windows SDK 8.1+ headers
-#endif
-
 // Forward declare message handler from imgui_impl_win32.cpp
 extern IMGUI_IMPL_API LRESULT ImGui_ImplWin32_WndProcHandler(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
 
@@ -282,15 +278,6 @@ LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
     case WM_DESTROY:
         ::PostQuitMessage(0);
         return 0;
-    case WM_DPICHANGED:
-        if (ImGui::GetIO().ConfigFlags & ImGuiConfigFlags_DpiEnableScaleViewports)
-        {
-            //const int dpi = HIWORD(wParam);
-            //printf("WM_DPICHANGED to %d (%.0f%%)\n", dpi, (float)dpi / 96.0f * 100.0f);
-            const RECT* suggested_rect = (RECT*)lParam;
-            ::SetWindowPos(hWnd, nullptr, suggested_rect->left, suggested_rect->top, suggested_rect->right - suggested_rect->left, suggested_rect->bottom - suggested_rect->top, SWP_NOZORDER | SWP_NOACTIVATE);
-        }
-        break;
     }
     return ::DefWindowProcW(hWnd, msg, wParam, lParam);
 }

imgui.h

@@ -1666,7 +1666,7 @@ enum ImGuiConfigFlags_
     // [BETA] Viewports
     // When using viewports it is recommended that your default value for ImGuiCol_WindowBg is opaque (Alpha=1.0) so transition to a viewport won't be noticeable.
     ImGuiConfigFlags_ViewportsEnable        = 1 << 10,  // Viewport enable flags (require both ImGuiBackendFlags_PlatformHasViewports + ImGuiBackendFlags_RendererHasViewports set by the respective backends)
-    ImGuiConfigFlags_DpiEnableScaleViewports= 1 << 14,  // [BETA: Don't use] FIXME-DPI: Reposition and resize imgui windows when the DpiScale of a viewport changed (mostly useful for the main viewport hosting other window). Note that resizing the main window itself is up to your application.
+    ImGuiConfigFlags_DpiEnableScaleViewports= 1 << 14,  // [BETA: Don't use] FIXME-DPI: Reposition and resize imgui and platform windows when the DpiScale of a viewport changed.
     ImGuiConfigFlags_DpiEnableScaleFonts    = 1 << 15,  // [BETA: Don't use] FIXME-DPI: Request bitmap-scaled fonts to match DpiScale. This is a very low-quality workaround. The correct way to handle DPI is _currently_ to replace the atlas and/or fonts in the Platform_OnChangedViewport callback, but this is all early work in progress.
 
     // User storage (to allow your backend/engine to communicate to code that may be shared between multiple projects. Those flags are NOT used by core Dear ImGui)