usa stimulus check

My understanding is that you do not need to have filed a 2019 1040.  They will take the adjusted gross income off your 2018 form and work from that.  I have to wonder about this, since I hold investments:

2018 AG Income was lower due to the December downturn of the stock market. They would send us $1200 for a married couple.  2019 will be high enough that they would not give us anything under preset guidelines.  If they send us a check, will they then ask for it back once my 2019 return is filed?

This would not surprise me a bit, and in fact the IRS would probably ask for interest even though interest rates are near zero.  I hardly expect sympathy, since most of us squeak without benefit of investments, but it is an interesting, catch-22 type question.  It's rather typical of navigating US tax challenges.