React to WM_NCACTIVATE instead of WM_SETFOCUS or WM_ACTIVATE for focus changes

See https://github.com/libsdl-org/SDL/pull/4293 and https://github.com/libsdl-org/SDL/issues/4450 for details

src/video/windows/SDL_windowsevents.c

@@ -369,6 +369,54 @@ WIN_CheckAsyncMouseRelease(SDL_WindowData *data)
     data->mouse_button_flags = 0;
 }
 
+static void
+WIN_UpdateFocus(SDL_Window *window)
+{
+    SDL_WindowData *data = (SDL_WindowData *) window->driverdata;
+    HWND hwnd = data->hwnd;
+
+    if (GetForegroundWindow() == hwnd) {
+        POINT cursorPos;
+
+        if (SDL_GetKeyboardFocus() != window) {
+            SDL_SetKeyboardFocus(window);
+        }
+
+        GetCursorPos(&cursorPos);
+        ScreenToClient(hwnd, &cursorPos);
+        SDL_SendMouseMotion(window, 0, 0, cursorPos.x, cursorPos.y);
+
+        WIN_CheckAsyncMouseRelease(data);
+        WIN_UpdateClipCursor(window);
+
+        /*
+         * FIXME: Update keyboard state
+         */
+        WIN_CheckClipboardUpdate(data->videodata);
+
+        SDL_ToggleModState(KMOD_CAPS, (GetKeyState(VK_CAPITAL) & 0x0001) != 0);
+        SDL_ToggleModState(KMOD_NUM, (GetKeyState(VK_NUMLOCK) & 0x0001) != 0);
+        SDL_ToggleModState(KMOD_SCROLL, (GetKeyState(VK_SCROLL) & 0x0001) != 0);
+
+    } else {
+        RECT rect;
+
+        data->in_window_deactivation = SDL_TRUE;
+
+        if (SDL_GetKeyboardFocus() == window) {
+            SDL_SetKeyboardFocus(NULL);
+            WIN_ResetDeadKeys();
+        }
+
+        if (GetClipCursor(&rect) && SDL_memcmp(&rect, &data->cursor_clipped_rect, sizeof(rect)) == 0) {
+            ClipCursor(NULL);
+            SDL_zero(data->cursor_clipped_rect);
+        }
+
+        data->in_window_deactivation = SDL_FALSE;
+    }
+}
+
 static BOOL
 WIN_ConvertUTF32toUTF8(UINT32 codepoint, char * text)
 {
@@ -658,6 +706,11 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
         {
             /* Don't immediately clip the cursor in case we're clicking minimize/maximize buttons */
             data->skip_update_clipcursor = SDL_TRUE;
+
+            /* Update the focus here, since it's possible to get WM_ACTIVATE and WM_SETFOCUS without
+               actually being the foreground window, but this appears to get called in all cases where
+               the global foreground window changes to and from this window. */
+            WIN_UpdateFocus(data->window);
         }
         break;
 
@@ -687,55 +740,8 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
                 }
 
                 SDL_SendWindowEvent(data->window, SDL_WINDOWEVENT_SHOWN, 0, 0);
-            }
-        }
-        returnCode = 0;
-        break;
 
-    case WM_SETFOCUS:
-        {
-            POINT cursorPos;
-
-            if (SDL_GetKeyboardFocus() != data->window) {
-                SDL_SetKeyboardFocus(data->window);
             }
-
-            GetCursorPos(&cursorPos);
-            ScreenToClient(hwnd, &cursorPos);
-            SDL_SendMouseMotion(data->window, 0, 0, cursorPos.x, cursorPos.y);
-
-            WIN_CheckAsyncMouseRelease(data);
-            WIN_UpdateClipCursor(data->window);
-
-            /*
-             * FIXME: Update keyboard state
-             */
-            WIN_CheckClipboardUpdate(data->videodata);
-
-            SDL_ToggleModState(KMOD_CAPS, (GetKeyState(VK_CAPITAL) & 0x0001) != 0);
-            SDL_ToggleModState(KMOD_NUM, (GetKeyState(VK_NUMLOCK) & 0x0001) != 0);
-            SDL_ToggleModState(KMOD_SCROLL, (GetKeyState(VK_SCROLL) & 0x0001) != 0);
-        }
-        returnCode = 0;
-        break;
-
-    case WM_KILLFOCUS:
-        {
-            RECT rect;
-
-            data->in_window_deactivation = SDL_TRUE;
-
-            if (SDL_GetKeyboardFocus() == data->window) {
-                SDL_SetKeyboardFocus(NULL);
-                WIN_ResetDeadKeys();
-            }
-
-            if (GetClipCursor(&rect) && SDL_memcmp(&rect, &data->cursor_clipped_rect, sizeof(rect)) == 0) {
-                ClipCursor(NULL);
-                SDL_zero(data->cursor_clipped_rect);
-            }
-
-            data->in_window_deactivation = SDL_FALSE;
         }
         returnCode = 0;
         break;