What ‘m looking for is a trick to crop a buffer. That means to cut off the zero parts at the beginning and the end after recording.
Looking around I only did find St.hreshrec, which unfortunately only records mono. (It crops directly after recording the buffer.)
There is a ‘crop’ message to buffer~. The trick is figuring out where to crop.
I recall doing something like this before with peek~ and cropping from when the value of the buffer sample index stops being 0. after a certain amount of samples, then finishing when the signal value returns to being 0. for a certain amount of samples.