is there a reason why the capture window can't 'pop up' by default?
I'm not sure what you mean by the capture window.. SC has 3 basic options for what to do after a screenshot. It can pop up a small post-capture dialog to show you a mini version of the screenshot and give you some choices. Or it can restore it's main window to let you edit the screenshot, etc. Or it can stay minimized.
The difficulty in what you are requesting is simply that it is basically asking for a 4th option -- you want to pop-up the capture, but in a way that closes if you click outside of it, and you want it to be sized for the capture (at least when feasible). I'm thinking it would be easier to write/use a standalone viewer for that and just tell SC to launch it after each capture and keep itself minimized.