I suppose they could implement smooth panning in high fps even if actual updates would be slower… though it might look funky.

If you can do that, you already had enough space for reflinking not to matter in the first place, right? Or you can carefully do defragmenting in parts, running dupremove incrementally? seems like a lot of wasted time :).

Here’s one sharp edge: defrag will unshare file contents so sometimes it’s not just feasible to do it.