Center the window on resize-to-fit

When removing the black borders (by double-clicking on them, or by
pressing MOD+w), the window is resized to fit the device screen, but its
top-left position was left unchanged.

Instead, move the window so that the new window area is at the center of
the old window area.

Refs #2387 <#2387>

app/src/screen.c

@@ -41,6 +41,18 @@ get_window_size(const struct screen *screen) {
     return size;
 }
 
+static struct point
+get_window_position(const struct screen *screen) {
+    int x;
+    int y;
+    SDL_GetWindowPosition(screen->window, &x, &y);
+
+    struct point point;
+    point.x = x;
+    point.y = y;
+    return point;
+}
+
 // set the window size to be applied when fullscreen is disabled
 static void
 set_window_size(struct screen *screen, struct size new_size) {
@@ -122,13 +134,6 @@ get_optimal_size(struct size current_size, struct size content_size) {
     return window_size;
 }
 
-// same as get_optimal_size(), but read the current size from the window
-static inline struct size
-get_optimal_window_size(const struct screen *screen, struct size content_size) {
-    struct size window_size = get_window_size(screen);
-    return get_optimal_size(window_size, content_size);
-}
-
 // initially, there is no current size, so use the frame size as current size
 // req_width and req_height, if not 0, are the sizes requested by the user
 static inline struct size
@@ -662,9 +667,20 @@ screen_resize_to_fit(struct screen *screen) {
         return;
     }
 
+    struct point point = get_window_position(screen);
+    struct size window_size = get_window_size(screen);
+
     struct size optimal_size =
-        get_optimal_window_size(screen, screen->content_size);
+        get_optimal_size(window_size, screen->content_size);
+
+    // Center the window related to the device screen
+    assert(optimal_size.width <= window_size.width);
+    assert(optimal_size.height <= window_size.height);
+    uint32_t new_x = point.x + (window_size.width - optimal_size.width) / 2;
+    uint32_t new_y = point.y + (window_size.height - optimal_size.height) / 2;
+
     SDL_SetWindowSize(screen->window, optimal_size.width, optimal_size.height);
+    SDL_SetWindowPosition(screen->window, new_x, new_y);
     LOGD("Resized to optimal size: %ux%u", optimal_size.width,
                                            optimal_size.height);
 }