Improve move/resize visual smoothness on Windows

Fixes https://github.com/libsdl-org/SDL/issues/12528

(cherry picked from commit 6b13d69105721ac54cd7224b0493065c57776c89)

src/video/windows/SDL_windowsevents.c

@@ -1859,6 +1859,13 @@ LRESULT CALLBACK WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lPara
     {
         if (wParam == (UINT_PTR)SDL_IterateMainCallbacks) {
             SDL_OnWindowLiveResizeUpdate(data->window);
+
+#if !defined(SDL_PLATFORM_XBOXONE) && !defined(SDL_PLATFORM_XBOXSERIES)
+            // Make sure graphics operations are complete for smooth refresh
+            if (data->videodata->DwmFlush) {
+                data->videodata->DwmFlush();
+            }
+#endif
             return 0;
         }
     } break;

src/video/windows/SDL_windowsvideo.c

@@ -108,6 +108,9 @@ static void WIN_DeleteDevice(SDL_VideoDevice *device)
     if (data->shcoreDLL) {
         SDL_UnloadObject(data->shcoreDLL);
     }
+    if (data->dwmapiDLL) {
+        SDL_UnloadObject(data->dwmapiDLL);
+    }
 #endif
 #ifdef HAVE_DXGI_H
     if (data->pDXGIFactory) {
@@ -184,6 +187,17 @@ static SDL_VideoDevice *WIN_CreateDevice(void)
     } else {
         SDL_ClearError();
     }
+
+    data->dwmapiDLL = SDL_LoadObject("DWMAPI.DLL");
+    if (data->dwmapiDLL) {
+        /* *INDENT-OFF* */ // clang-format off
+        data->DwmFlush = (HRESULT (WINAPI *)(void))SDL_LoadFunction(data->dwmapiDLL, "DwmFlush");
+        data->DwmEnableBlurBehindWindow = (HRESULT (WINAPI *)(HWND hwnd, const DWM_BLURBEHIND *pBlurBehind))SDL_LoadFunction(data->dwmapiDLL, "DwmEnableBlurBehindWindow");
+        data->DwmSetWindowAttribute = (HRESULT (WINAPI *)(HWND hwnd, DWORD dwAttribute, LPCVOID pvAttribute, DWORD cbAttribute))SDL_LoadFunction(data->dwmapiDLL, "DwmSetWindowAttribute");
+        /* *INDENT-ON* */ // clang-format on
+    } else {
+        SDL_ClearError();
+    }
 #endif // #if !defined(SDL_PLATFORM_XBOXONE) && !defined(SDL_PLATFORM_XBOXSERIES)
 
 #ifdef HAVE_DXGI_H

src/video/windows/SDL_windowsvideo.h

@@ -374,6 +374,45 @@ typedef struct tagINPUTCONTEXT2
 } INPUTCONTEXT2, *PINPUTCONTEXT2, NEAR *NPINPUTCONTEXT2, FAR *LPINPUTCONTEXT2;
 #endif
 
+// Corner rounding support  (Win 11+)
+#ifndef DWMWA_WINDOW_CORNER_PREFERENCE
+#define DWMWA_WINDOW_CORNER_PREFERENCE 33
+#endif
+typedef enum {
+    DWMWCP_DEFAULT = 0,
+    DWMWCP_DONOTROUND = 1,
+    DWMWCP_ROUND = 2,
+    DWMWCP_ROUNDSMALL = 3
+} DWM_WINDOW_CORNER_PREFERENCE;
+
+// Border Color support (Win 11+)
+#ifndef DWMWA_BORDER_COLOR
+#define DWMWA_BORDER_COLOR 34
+#endif
+
+#ifndef DWMWA_COLOR_DEFAULT
+#define DWMWA_COLOR_DEFAULT 0xFFFFFFFF
+#endif
+
+#ifndef DWMWA_COLOR_NONE
+#define DWMWA_COLOR_NONE 0xFFFFFFFE
+#endif
+
+// Transparent window support
+#ifndef DWM_BB_ENABLE
+#define DWM_BB_ENABLE 0x00000001
+#endif
+#ifndef DWM_BB_BLURREGION
+#define DWM_BB_BLURREGION 0x00000002
+#endif
+typedef struct
+{
+    DWORD flags;
+    BOOL enable;
+    HRGN blur_region;
+    BOOL transition_on_maxed;
+} DWM_BLURBEHIND;
+
 // Private display data
 
 struct SDL_VideoData
@@ -420,6 +459,11 @@ struct SDL_VideoData
     BOOL (WINAPI *GetPointerType)(UINT32 pointerId, POINTER_INPUT_TYPE *pointerType);
     BOOL (WINAPI *GetPointerPenInfo)(UINT32 pointerId, POINTER_PEN_INFO *penInfo);
 
+    SDL_SharedObject *dwmapiDLL;
+    /* *INDENT-OFF* */ // clang-format off
+    HRESULT (WINAPI *DwmFlush)(void);
+    HRESULT (WINAPI *DwmEnableBlurBehindWindow)(HWND hwnd, const DWM_BLURBEHIND *pBlurBehind);
+    HRESULT (WINAPI *DwmSetWindowAttribute)(HWND hwnd, DWORD dwAttribute, LPCVOID pvAttribute, DWORD cbAttribute);
     /* *INDENT-ON* */ // clang-format on
 #endif                // !defined(SDL_PLATFORM_XBOXONE) && !defined(SDL_PLATFORM_XBOXSERIES)
 

src/video/windows/SDL_windowswindow.c

@@ -38,10 +38,6 @@
 // Dropfile support
 #include <shellapi.h>
 
-// DWM setting support
-typedef HRESULT (WINAPI *DwmSetWindowAttribute_t)(HWND hwnd, DWORD dwAttribute, LPCVOID pvAttribute, DWORD cbAttribute);
-typedef HRESULT (WINAPI *DwmGetWindowAttribute_t)(HWND hwnd, DWORD dwAttribute, PVOID pvAttribute, DWORD cbAttribute);
-
 // Dark mode support
 typedef enum {
     UXTHEME_APPMODE_DEFAULT,
@@ -80,46 +76,6 @@ typedef UxthemePreferredAppMode (WINAPI *SetPreferredAppMode_t)(UxthemePreferred
 typedef BOOL (WINAPI *SetWindowCompositionAttribute_t)(HWND, const WINDOWCOMPOSITIONATTRIBDATA *);
 typedef void (NTAPI *RtlGetVersion_t)(NT_OSVERSIONINFOW *);
 
-// Corner rounding support  (Win 11+)
-#ifndef DWMWA_WINDOW_CORNER_PREFERENCE
-#define DWMWA_WINDOW_CORNER_PREFERENCE 33
-#endif
-typedef enum {
-    DWMWCP_DEFAULT = 0,
-    DWMWCP_DONOTROUND = 1,
-    DWMWCP_ROUND = 2,
-    DWMWCP_ROUNDSMALL = 3
-} DWM_WINDOW_CORNER_PREFERENCE;
-
-// Border Color support (Win 11+)
-#ifndef DWMWA_BORDER_COLOR
-#define DWMWA_BORDER_COLOR 34
-#endif
-
-#ifndef DWMWA_COLOR_DEFAULT
-#define DWMWA_COLOR_DEFAULT 0xFFFFFFFF
-#endif
-
-#ifndef DWMWA_COLOR_NONE
-#define DWMWA_COLOR_NONE 0xFFFFFFFE
-#endif
-
-// Transparent window support
-#ifndef DWM_BB_ENABLE
-#define DWM_BB_ENABLE 0x00000001
-#endif
-#ifndef DWM_BB_BLURREGION
-#define DWM_BB_BLURREGION 0x00000002
-#endif
-typedef struct
-{
-    DWORD flags;
-    BOOL enable;
-    HRGN blur_region;
-    BOOL transition_on_maxed;
-} DWM_BLURBEHIND;
-typedef HRESULT(WINAPI *DwmEnableBlurBehindWindow_t)(HWND hwnd, const DWM_BLURBEHIND *pBlurBehind);
-
 // Windows CE compatibility
 #ifndef SWP_NOCOPYBITS
 #define SWP_NOCOPYBITS 0
@@ -744,6 +700,7 @@ static void WIN_SetKeyboardFocus(SDL_Window *window, bool set_active_focus)
 
 bool WIN_CreateWindow(SDL_VideoDevice *_this, SDL_Window *window, SDL_PropertiesID create_props)
 {
+    SDL_VideoData *videodata = _this->internal;
     HWND hwnd = (HWND)SDL_GetPointerProperty(create_props, SDL_PROP_WINDOW_CREATE_WIN32_HWND_POINTER, SDL_GetPointerProperty(create_props, "sdl2-compat.external_window", NULL));
     HWND parent = NULL;
     if (hwnd) {
@@ -805,24 +762,19 @@ bool WIN_CreateWindow(SDL_VideoDevice *_this, SDL_Window *window, SDL_Properties
 #if !defined(SDL_PLATFORM_XBOXONE) && !defined(SDL_PLATFORM_XBOXSERIES)
     // FIXME: does not work on all hardware configurations with different renders (i.e. hybrid GPUs)
     if (window->flags & SDL_WINDOW_TRANSPARENT) {
-        SDL_SharedObject *handle = SDL_LoadObject("dwmapi.dll");
-        if (handle) {
-            DwmEnableBlurBehindWindow_t DwmEnableBlurBehindWindowFunc = (DwmEnableBlurBehindWindow_t)SDL_LoadFunction(handle, "DwmEnableBlurBehindWindow");
-            if (DwmEnableBlurBehindWindowFunc) {
-                /* The region indicates which part of the window will be blurred and rest will be transparent. This
-                   is because the alpha value of the window will be used for non-blurred areas
-                   We can use (-1, -1, 0, 0) boundary to make sure no pixels are being blurred
-                */
-                HRGN rgn = CreateRectRgn(-1, -1, 0, 0);
-                DWM_BLURBEHIND bb;
-                bb.flags = (DWM_BB_ENABLE | DWM_BB_BLURREGION);
-                bb.enable = TRUE;
-                bb.blur_region = rgn;
-                bb.transition_on_maxed = FALSE;
-                DwmEnableBlurBehindWindowFunc(hwnd, &bb);
-                DeleteObject(rgn);
-            }
-            SDL_UnloadObject(handle);
+        if (videodata->DwmEnableBlurBehindWindow) {
+            /* The region indicates which part of the window will be blurred and rest will be transparent. This
+               is because the alpha value of the window will be used for non-blurred areas
+               We can use (-1, -1, 0, 0) boundary to make sure no pixels are being blurred
+            */
+            HRGN rgn = CreateRectRgn(-1, -1, 0, 0);
+            DWM_BLURBEHIND bb;
+            bb.flags = (DWM_BB_ENABLE | DWM_BB_BLURREGION);
+            bb.enable = TRUE;
+            bb.blur_region = rgn;
+            bb.transition_on_maxed = FALSE;
+            videodata->DwmEnableBlurBehindWindow(hwnd, &bb);
+            DeleteObject(rgn);
         }
     }
 
@@ -1264,29 +1216,19 @@ void WIN_RestoreWindow(SDL_VideoDevice *_this, SDL_Window *window)
     }
 }
 
-static void WIN_UpdateCornerRoundingForHWND(HWND hwnd, DWM_WINDOW_CORNER_PREFERENCE cornerPref)
+static void WIN_UpdateCornerRoundingForHWND(SDL_VideoDevice *_this, HWND hwnd, DWM_WINDOW_CORNER_PREFERENCE cornerPref)
 {
-    SDL_SharedObject *handle = SDL_LoadObject("dwmapi.dll");
-    if (handle) {
-        DwmSetWindowAttribute_t DwmSetWindowAttributeFunc = (DwmSetWindowAttribute_t)SDL_LoadFunction(handle, "DwmSetWindowAttribute");
-        if (DwmSetWindowAttributeFunc) {
-            DwmSetWindowAttributeFunc(hwnd, DWMWA_WINDOW_CORNER_PREFERENCE, &cornerPref, sizeof(cornerPref));
-        }
-
-        SDL_UnloadObject(handle);
+    SDL_VideoData *videodata = _this->internal;
+    if (videodata->DwmSetWindowAttribute) {
+        videodata->DwmSetWindowAttribute(hwnd, DWMWA_WINDOW_CORNER_PREFERENCE, &cornerPref, sizeof(cornerPref));
     }
 }
 
-static void WIN_UpdateBorderColorForHWND(HWND hwnd, COLORREF colorRef)
+static void WIN_UpdateBorderColorForHWND(SDL_VideoDevice *_this, HWND hwnd, COLORREF colorRef)
 {
-    SDL_SharedObject *handle = SDL_LoadObject("dwmapi.dll");
-    if (handle) {
-        DwmSetWindowAttribute_t DwmSetWindowAttributeFunc = (DwmSetWindowAttribute_t)SDL_LoadFunction(handle, "DwmSetWindowAttribute");
-        if (DwmSetWindowAttributeFunc) {
-            DwmSetWindowAttributeFunc(hwnd, DWMWA_BORDER_COLOR, &colorRef, sizeof(colorRef));
-        }
-
-        SDL_UnloadObject(handle);
+    SDL_VideoData *videodata = _this->internal;
+    if (videodata->DwmSetWindowAttribute) {
+        videodata->DwmSetWindowAttribute(hwnd, DWMWA_BORDER_COLOR, &colorRef, sizeof(colorRef));
     }
 }
 
@@ -1353,13 +1295,13 @@ SDL_FullscreenResult WIN_SetWindowFullscreen(SDL_VideoDevice *_this, SDL_Window
         }
 
         // Disable corner rounding & border color (Windows 11+) so the window fills the full screen
-        WIN_UpdateCornerRoundingForHWND(hwnd, DWMWCP_DONOTROUND);
-        WIN_UpdateBorderColorForHWND(hwnd, DWMWA_COLOR_NONE);
+        WIN_UpdateCornerRoundingForHWND(_this, hwnd, DWMWCP_DONOTROUND);
+        WIN_UpdateBorderColorForHWND(_this, hwnd, DWMWA_COLOR_NONE);
     } else {
         BOOL menu;
 
-        WIN_UpdateCornerRoundingForHWND(hwnd, DWMWCP_DEFAULT);
-        WIN_UpdateBorderColorForHWND(hwnd, DWMWA_COLOR_DEFAULT);
+        WIN_UpdateCornerRoundingForHWND(_this, hwnd, DWMWCP_DEFAULT);
+        WIN_UpdateBorderColorForHWND(_this, hwnd, DWMWA_COLOR_DEFAULT);
 
         /* Restore window-maximization state, as applicable.
            Special care is taken to *not* do this if and when we're