@sdetweil
Of course, there are many tricks to display something to specific region.
.fullscreen.below is the lowest layer of region system of MM. its position is already absolute and fit for screen size. If you put some module here, it will be displayed like background.
.fullscreen.above is the highest layer. If you put some module here, it will cover the lower region layer.
And, modifying CSS is safer and easier option than hardcoded in the module itself. Therefore there is “custom.css” for customization.
The main reason you are confused might be this;
In main.css
body {
margin: 60px;
position: absolute;
height: calc(100% - 120px);
width: calc(100% - 120px);
It gives 60px margin for the entire screen. So YOUR_FULLSCREEN module couldn’t cover the WHOLE_SCREEN_YOU_THOUGHT. But I recommend you leave it. Because, some ppl needs those margin for their mirror edge. Who don’t need that margin will modify his custom.css for using fullscreen without margin by himself.
I think you’d better to research some BACKGROUND-IMAGE modules to get a hint to use full screen CSS. I believe it’s enough to put your module with 100% width/height into .fullscreen.below or .fullscreen.above to display your module as full screen size.