This works for me on niri.

Hey, sorry for the slow response on this and thanks for the PR! Unfortunately this issue isn't quite so simple.

The reason opposing anchors were set to centre the window, rather than neither, was because this is required to get a full-width / height window on a compositor that:

• Supports fractional scaling
• Does not support the wp-fractional-scale-v1 protocol.
  In this scenario, it's impossible (I think) for the window to determine how big it needs to be, at least in the way tofi works.

With this patch set, setting --width=0 --anchor=center on sway gives me:

zwlr_layer_surface_v1#20: error 1: width 0 requested without setting left and right anchors

and no visible window.

Back when I was last updating tofi, wp-fractional-scale-v1 was not widely implemented, so I couldn't rely on it.
Nowadays however, it seems almost all compositors support it, so we can probably get rid of the legacy --width=0 behaviour. I think the correct thing to do is:

• Make a release (soon!), with warnings that --width=0 and --height=0 are deprecated, and 100% is preferred.
• In the release after that, remove --width=0 support and merge this.