YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp> writes:

>> An earlier version of emacs-mac-app had the ability to fill
>> ½-fullscreen on long press of the maximize button. The current
>> version does not do that.
>> 
>> --->  Installing emacs-mac-app @7.8_1+rsvg
>> --->  Activating emacs-mac-app @7.8_1+rsvg
>
> Works here.  Which version of macOS are you using?
> Is it reproducible without any customizations?
> (say, $ open /Applications/MacPorts/EmacsMac.app --args -Q)

Starting EmacsMac.app as suggested allows the expected, desired
behavior.

The macOS version string is
GNU Emacs 26.3 (build 1, x86_64-apple-darwin15.6.0, Carbon Version 157 AppKit 1404.47)
 of 2019-12-04

Something in my customization has introduced the undesired behavior.