A square that keeps the whole picture
The usual way to square a photo cuts the long side until it matches the short one — and that's where heads and edges get lost. This grows the short side to meet the long one instead, so the result is 1:1 and nothing is trimmed.
Mirror fill is the default because a reflected border blends naturally on most square posts; the blurred backdrop and the edge painter are there for busier scenes.
Questions
Does my photo get uploaded?
No. The picture is read, grown and encoded by code running in this browser tab. It is never sent to a server — there is no server to send it to. The only download that ever happens is the optional edge-painter model, which travels one way, from our model file server to you.
Why not crop to a square?
Cropping to a square throws away the long edges, which is exactly what people searching "square without cropping" want to avoid. This keeps all of it and builds the short side out.
Is this good for Instagram?
For a 1:1 post, yes. For 4:5 or 9:16, the Instagram route opens on those shapes directly.