There is a typo in Handbook's chapter 6.5. The Sway Compositor The line output "My Workstation" mode 1366x786@60Hz position 1366 0 most likely it should contain 768 and not 786
Seems correct. Wikipedia has this list of graphics modes: https://en.wikipedia.org/wiki/List_of_common_display_resolutions and 1366x768 is the correct entry.
This seems to have been fixed by now: https://reviews.freebsd.org/D47101 which resulted in commit https://cgit.freebsd.org/doc/commit/?id=0bda34e56fb402911e680dfe0b487985b042ac37. Thanks for filing the PR anyway.