I can zoom in on the top corner of a square wave, too...
And then add an FFT of that...
That "vertical zoom" via a math function can be used in a pinch, but it is much less convenient than Siglent's interactive zoom. Siglent lets you use the regular vertical (and horizontal) controls to adjust the magnification and position of the zoomed-in section. And it indicates the zoomed-in area as a nice little "you are here" rectangle in the upper, non-zoomed trace, so you don't lose track of where you are looking.
Combining FFT with Zoom mode is possible in the Siglent UI as well. Let me just say that I have found very few practical use cases where I really want to see the trace in detail and look at the FFT at the same time. To get decent FFT resolution, I typically want to capture a long signal segment, hence can't resolve any detail along the time axis anyway. I merely use the trace display to make sure I have chosen a good vertical amplification -- and then often disable it to see the FFT exclusively. (Not sure whether the Rigol UI allows for that?)
Edit: Adding a screenshot which shows how a combined Zoom + FFT looks on the SDS800X HD, to give a better idea. Note the "you are here" rectangle in the upper trace window.
As a side note: I find it much easier to get meaningful, "clean" axis tick values on the Siglent. The dual Flexknobs in Rigol's UI, with their dynamic assignment of functions, are a great idea IMO. But Rigol implemented them in an awkward way -- they typically change values in very odd increments, often on a scale which does not even include the even values. So you either use the on-screen keyboard
a lot to set axis scales and offsets, or you end up with very odd values as shown in Fungus' screenshots above. Easily changed in firmware; I hope Rigol will do that at some point to make the Flexknobs more useful.