The likelihood of any legal restriction was probably close to zero - it’s only from today’s era of hyper-regulation that we might even imagine something like that.
Most likely it was a deliberate technical limitation. After all, dialog windows themselves were already overlapped. I remember well what a headache it was to program and render graphical elements on those old machines (PC AT 80286 with 256 KB of RAM).
Most likely it was a deliberate technical limitation. After all, dialog windows themselves were already overlapped. I remember well what a headache it was to program and render graphical elements on those old machines (PC AT 80286 with 256 KB of RAM).