Just so everyone's clear on what this example is doing ... the user's "map" is the rectangular black polygon (they specified 5 lat/long coordinates in their map dataset), and then Proc Sgmap automatically determines which Openstreetmap tiles to put behind it. I'm not 100% certain whether Sgmap has to use whole tiles, or if it 'trims' the tiles. In this case the black rectangle is the user's map polygon, not the border of the page. None of these details shed any light on the user's actual question, but thought I'd explain, just in case you were curious or confused.
Per the user's question about having the Sgmap go all the way to the edge of the available space, so that there's absolutely no "white space" around the edge ... I don't know of any way to do that. I'm not saying it's "impossible" but I don't know of a way.
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