SmartSDR horizontal scroll spectrum lags?

I think what you're seeing is the effect of tuning that is happening at multiple points in the DSP chains and not all of them are synchronous with the Panadapter data tap. This means that some of the data shows up prior to tuning and some of after. It has a spreading effect. This effect is compounded by the fact that waterfall data is not one sample, it is many samples over a period of time integrated together. We could attempt to prevent this by not collecting data while inside a tuning window and throwing out any accumulated data (average), but I think that effect would actually be worse visually.

In an ideal world all of the tuning would be done in between frames of incoming data and then we could do a better job of cleaning up these rough edges, but this is a lot harder than it initially appears. Just ask anyone that has ever worked on a synchronous system with multiple receivers/oscillators.

In short, what others have is correct -- this is "normal" behavior with today's software (SmartSDR v3.5.9).