Per an earlier comment:
.. it’s a known missing feature, planned for a future release.
I agree, it is a nice feature that I often use w/my logic before and miss having it with the MSO version. It is especially missed for trimming storage demands for save files, as MSO captures can use a lot more data than before. At least you can tune the sample frequency to help reduce file size, but I’d much rather have a larger initial snapshot that I can crop/trim afterwards vs. wasting time tweaking the trigger & buffer length for the right snapshot ‘pan & zoom’ ahead of time.
Meanwhile, have you tried adjusting the logic threshold to filter out the noise, or inspected the signal with analog channel? If only while ‘idle’ did you include chip select signal(s) to only decode when the SPI bus is active and a chip select is asserted?
Finally here’s, the full thread (quoted above), with work arounds for some other scenarios: