In LVGL 9.1, lv_sdl_window_set_zoom(lv_display_t disp, uint8_t zoom) only allows integer zoom values >= 1.
This means you can zoom in a small window to a larger size, but cannot zoom out a large window to a smaller size. Changing the zoom value to a double would allow for zooming out and allow more granularity in resizing the window. This was possible in LVGL 8.4.