Casting puts a shrunken copy of the page along one edge. The bright band is where you are, and dragging it travels the page.
Good to know
The map is built in the browser out of the page itself. No picture is taken and nothing is sent anywhere.
It follows whichever thing actually scrolls, the window or a panel inside it, and redraws as a page grows. Esc or the × puts it away, as does casting it again.
The map is a copy of what's in the page, so a page that builds itself as you look at it can come out sparse or empty, and anything inside a frame or a component's own shadow won't appear. Parts pinned to the window sit at the top of the map rather than where you're reading.
It's a snapshot, redrawn a second or two after the page changes, so somewhere busy will always be slightly behind. When the map shows the wrong thing, the ⌗ in its header lets you pick which part of the page it maps.
The side of the window the map sits against.
Pushing narrows the page so nothing is left underneath the map. Parts pinned to the window, like a fixed header, stay where they are. This picks how it opens; the button along the bottom swaps between them whenever you like.