You can optionally set a view mode in the <form> tag to start Smart Checkout in fullscreen mode.
These are the options:
layer: If possible, Smart Checkout opens in a modal layer above the shop page. Some browsers need Smart Checkout to start in fullscreen mode due to restrictions for cross-origin ressource sharing (CORS). There are also reasons for Smart Checkout to switch to fullscreen mode even if it started as layer.
fullscreen: Forces Smart Checkout to open in fullscreen mode from the beginning.
The default view mode is layer.