Stack videos with ffmpeg without padding The hstack feature works great, but it pads the videos. That is, it puts black space between them. I use a command like this:

ffmpeg -i 1.mp4 -i 2.mp4  -filter_complex "hstack" out.mp4


How do I remove the black space separating the component videos?

Perhaps related, how do I control the size of the output video? If I could change the width of the output video, I imagine that would make the padding go away. But I couldn't find a way to combine the scale= parameter with hstack.
The stack filters don't insert any padding. Your videos probably have empty space in them. You'll need the crop filter to remove the dead space.

The stack filters require the video edge along the join axis to have the same dimensions i.e. height for hstack and width for vstack. So, don't crop those aspects.