[FFmpeg-user] Scaling a video with multiple different resolutions

[Alex Speller]

I have a webm file that has multiple different resolutions in it (it's a
screen capture of a window that changes dimensions).

https://www.dropbox.com/s/ptueirabmmht0fr/4be7fdb7-d7e9-41b4-ba26-e20a3eeb6026.webm?dl=0

Is there any way to "normalize" the dimensions and output a video of a
constant resolution, having e.g. black borders around the video when the
resolutions change

If you play the video on the dropbox page above, in chrome or firefox, how
that looks is what I'd like to output. Tried messing around with scale,
setsar and setdar parameters but I can't figure out how to get output
that's not just distorted

Any help appreciated, thanks!