My 2 cents/comments:
- tab solves problem of visibility to all players
- I do not think players want to see it all the time. It's a pretty discrete action when I'm playing and am like "oh, what did I purchase??" Having the tab be always visible of course solves this, but the purchase is not necessarily that salient where I want to actually see it all the time.
In terms of 'hiding' the display, I would not go for a menu option. IMO menu options are more for options that are 'set-and-forget'. The purchase panel perhaps would get in the way, so you'd want to hide it in case you want to review the units that have moved or find one to undo. My thinking for 'hiding' the panel would be a sliding panel 'JSplitPane'. This would be something available perhaps on the edge of the map and slide out horizontally. If displayed, it would reduce the amount of map visible, clicking the slider hides/reveals it, and dragging would increase/decrease it's size (a bit similar to how the download maps window has a download progress area in a split pane, though the panel would be available instead as a brand new tab on the edge of the map). IMO I suspect going with a classic new tab is probably the easiest route for now.