win32: Hide the borders when showing a fullscreen window

If it is known that the window will immediately enter fullscreen upon being shown, set the borderless style when showing the window to hide the borders, or they may linger in the background if the client takes some time to draw the first frame.

Unnecessarily calling ShowWindow with SW_RESTORE when applying the window flags must be suppressed in this case, or the borders can reappear in a weird, partial state.

(cherry picked from commit d73fe0bc538b2fcfc143c94896df957b9ca3cb89)

src/video/windows/SDL_windowswindow.c

@@ -1032,8 +1032,9 @@ void WIN_GetWindowSizeInPixels(SDL_VideoDevice *_this, SDL_Window *window, int *
 
 void WIN_ShowWindow(SDL_VideoDevice *_this, SDL_Window *window)
 {
+    SDL_WindowData *data = window->internal;
+    HWND hwnd = data->hwnd;
     DWORD style;
-    HWND hwnd;
 
     bool bActivate = SDL_GetHintBoolean(SDL_HINT_WINDOW_ACTIVATE_WHEN_SHOWN, true);
 
@@ -1042,17 +1043,30 @@ void WIN_ShowWindow(SDL_VideoDevice *_this, SDL_Window *window)
         WIN_SetWindowPosition(_this, window);
     }
 
-    hwnd = window->internal->hwnd;
+    // If the window isn't borderless and will be fullscreen, use the borderless style to hide the initial borders.
+    if (window->pending_flags & SDL_WINDOW_FULLSCREEN) {
+        if (!(window->flags & SDL_WINDOW_BORDERLESS)) {
+            window->flags |= SDL_WINDOW_BORDERLESS;
+            style = GetWindowLong(hwnd, GWL_STYLE);
+            style &= ~STYLE_MASK;
+            style |= GetWindowStyle(window);
+            SetWindowLong(hwnd, GWL_STYLE, style);
+            window->flags &= ~SDL_WINDOW_BORDERLESS;
+        }
+    }
     style = GetWindowLong(hwnd, GWL_EXSTYLE);
     if (style & WS_EX_NOACTIVATE) {
         bActivate = false;
     }
+
+    data->showing_window = true;
     if (bActivate) {
         ShowWindow(hwnd, SW_SHOW);
     } else {
         // Use SetWindowPos instead of ShowWindow to avoid activating the parent window if this is a child window
         SetWindowPos(hwnd, NULL, 0, 0, 0, 0, window->internal->copybits_flag | SWP_SHOWWINDOW | SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER | SWP_NOOWNERZORDER);
     }
+    data->showing_window = false;
 
     if (window->flags & SDL_WINDOW_POPUP_MENU && bActivate) {
 	    WIN_SetKeyboardFocus(window, window->parent == SDL_GetKeyboardFocus());
@@ -1211,10 +1225,12 @@ void WIN_RestoreWindow(SDL_VideoDevice *_this, SDL_Window *window)
 {
     SDL_WindowData *data = window->internal;
     if (!(window->flags & SDL_WINDOW_FULLSCREEN)) {
-        HWND hwnd = data->hwnd;
-        data->expected_resize = true;
-        ShowWindow(hwnd, SW_RESTORE);
-        data->expected_resize = false;
+        if (!data->showing_window || window->flags & (SDL_WINDOW_MAXIMIZED | SDL_WINDOW_MINIMIZED)) {
+            HWND hwnd = data->hwnd;
+            data->expected_resize = true;
+            ShowWindow(hwnd, SW_RESTORE);
+            data->expected_resize = false;
+        }
     } else {
         data->windowed_mode_was_maximized = false;
     }

src/video/windows/SDL_windowswindow.h

@@ -84,6 +84,7 @@ struct SDL_WindowData
     bool in_window_deactivation;
     bool force_ws_maximizebox;
     bool disable_move_size_events;
+    bool showing_window;
     int in_modal_loop;
     RECT initial_size_rect;
     RECT cursor_clipped_rect; // last successfully committed clipping rect for this window