Fullscreen refactor pt. 3 turned out to be a refactor of a good chunk of core Smithay xdg-shell/layer-shell/session-lock logic, making it more correct. The fullscreen PR in niri now includes that Smithay refactor, and needs testing even more than before 
https://github.com/YaLTeR/niri/pull/2333#issuecomment-3263990200
Specifically, the refactor makes Smithay correctly track the last acked configure for each commit, also enforces the "must ack before committing first buffer" protocol rule.