I think I've figured it out.
At 5ms/div the FFT disappears, correctly because the sample rate is then too low.
But it comes back when enough sample rate is available - from 2ms.
But earlier it didn't come back and the scope was permanently stuck at 12.5MSsa/s.
So don't play around with it too much...
Otherwise, the FFT function behaves in the same way as its big brother, the 4204.
In other words, hardly anything can be set, no different modes, which is a particular pity that there is no averaging.
And what I already noticed in February with the 4204 is also present here.
Generator with wavecombine function, 10Mhz frequency, ch1 10Vpp, ch2 1mVpp 10.1Mhz frequency, 50Ohm termination.
I have done this with another scope with a "flattop" window and it works perfectly.
With Rigol it looks different when you switch from Hanning to Flattop.