Delphi is the easiest and most integrated way of writing cross platform UIs and it has been for a lot of years. It might not be sexy, but highly productive.

Delphi is the easiest and most integrated way of writing cross platform UIs and it has been for a lot of years. It might not be sexy, but highly productive. You can download and use a community edition of Delphi for free. The downside is insane licensing costs if you are making commercial software. I an not sure how far Lazarus has come but I believe it to have the same or similar abilities and if free and libre.