TwinBasic

[axisdj]

on the point of cross platform.

Speaking from experience, I have ported a vb6 app to macos using lazarus.

many of my controls including the flexgrid did port with minimal intervention.

I think cross platform can be achieved, but for most projects that will require some manual changes to port to cross platform user controls

I do not think its a show stopper at all.

added later:

to completely say it has to be one or the other is bad. if it initially works just in windows and then takes a few days/weeks to make it work on a another platform is fine and acceptable.