problem:
Reloading the configuration: SPC f e R
after changing the variable: dotspacemacs-fullscreen-at-startup
to: t
changes Emacs into fullscreen mode.
solution:
Only check the variable and change to fullscreen mode on startup,
before Spacemacs has been initialized.
Fixes: Reloading config with fullscreen-at-startup t (in macOS) toggles fullscreen mode #13073
444c6a7565