just installed 12.6.1 on MacOS El Capitan 10.11.6, upgraded from 12.5.2. Full screen mode no longer works. I click the green button, and it just (vaguely) maximizes the window, instead of entering full screen. (the old style green button behavior, like older OSX versions)
12.5.2 (and before, at least since 12.3 i think) worked fine in this regard.
EDIT: still broken in 12.7 on macOS 10.12.2 :(