Map display not filling window in QGIS on Mac?

I have the same (OS X 10.11, Qgis 3.2.2). What I'm doing is opening a new map window (View / New Map View), anchoring it to the top or side edge, resizing it and closing it again. This causes the original window to return to its normal size. Works for me.

This is a bug. Install an earlier version or wait for it to be fixed.

Update: This Mac-specific bug brought attention to the fact that the QGIS has not been as well supported as for Windows as for MacOS. There is now a MacOS specific bug fixing campaign.

I have compiled QGIS with Macports.

qgis3 3.2.1 (source) QGIS 3 is a user-friendly GIS based on Qt 5 Licenses: GPL-2+ Maintained by: vince atVeence openmaintainer Categories: gis Platforms: darwin Dependencies: cmake openssl qt5-qtbase bison expat fcgi gdal geos grass7 gsl ld64 libiconv libtasn1 libzip pkgconfig postgis2 postgresql10 proj py36-gdal py36-jinja2 py36-owslib py36-psycopg2 py36-pygments py36-pyqt5 py36-qscintilla-qt5 py36-requests py36-sip py36-six py36-yaml python36 qca-qt5 qca-qt5-ossl qjson-qt5 qt5-qtscript qt5-qtscxml qt5-qtwebkit qt5-qtxmlpatterns qt5-sqlite-plugin qtkeychain-qt5 qwt-qt5 spatialindex spatialite sqlite3 Variants: debug grass postgresql10 postgresql95 postgresql96 proj4 proj5 python34 python35 python36

The problem has been solved.