It’s always upper left corner, it’s the anker point and how much white you have around it depends on a couple things.
- Ratio between your visualization size (width x height) and the monitor you are using to display mappView;
- If you have zoom enable for your visualization, if enabled it would always scale to the size of used monitor regardless if the visualization is bigger or smaller than your display size. It also takes into account your tabs and bookmarks bar. So for proper alignment you would have to use fullscreen
The other option to size and center you visualization in a browser is to use the developer mode F12 and with some additional settings you can even emulate your actual screen size.
I posted a tutorial about this a while back under mappView Screen testing