My version of XnView (a screenshot taker, can do multiple captures) has been faithfully taking screenshots of things I was working on/my gameplay for months now, but prior to this evening has suddenly stopped doing these captures (in windowed mode). I got very frustrated a few nights ago, possibly on Sunday/Monday night.

But this evening I fixed this by going to the XnView console screen > Tools > Capture > and changed the Capture What from Active Window to Window (windows changes automatically depending on what windows is currently active) and once I did this, my XnView is back to faithfully doing its good old captures.