Since very early versions of Android, APIs existed for determining the characteristics of the hardware/software environment: What sensors exist, input methods, etc. If your app works on tablets and phones and handles orientation changes correctly, it will probably work in a window on a Fuchsia OS device.

It's about as much of a hassle than developing desktop app and less of a hassle than developing web apps. Meaning: really not that much of a hassle unless you're deliberately shooting yourself in a foot by doing equivalent of "position: absolute;" everywhere.

Which, for some reason, some iOS developers really loved doing which cause endless headaches when adding support to yet another form factor of those devices and endless "fragmentation" complaints.