Short legs, long stems.

Looking at a Haibike and according to the online sizeometer I need a small

Normally I have a medium as I like a long top tube but tried the small in the shop. Was a real joy not having my nether regions in daily peril when getting on.

Problem is the top  tube is at least 3cm shorter than any of my other bikes. It felt ok sitting in the shop but things only show up on a proper ride.

If I put an unfashionably long stem on it (80mm) will I be getting the best of both worlds or that horrible flip flop steering from days of old?

Look for another bike.
I have short legs for my height. I always show on manufacturers charts as a medium and had to put up with slightly short bikes for years. With old geometry a slightly longer stem wasn’t too bad but nowadays 50 to 80 is way too big a jump.

Plenty of modern bikes have much better stand over and I’ve been happily on large frames for years despite being below manufacturers recommendations.

Also, with dropper posts, less stand over clearance.  isn’t the issue it used to be.