A number of other (Py)Qt5-based projects are reporting that Qt windows/widgets aren't rendering properly under the recently-released macOS 11 ("Big Sur"). While I have not yet tested Big Sur myself, various sources allege that setting the environment variable QT_MAC_WANTS_LAYER=1 is sufficient to fix the problem (and is enabled by default in future (Py)Qt releases):

https://www.riverbankcomputing.com/pipermail/pyqt/2020-September/043192.html
https://www.riverbankcomputing.com/pipermail/pyqt/2020-November/043399.html
https://stackoverflow.com/questions/64833558/apps-not-popping-up-on-macos-big-sur-11-0-1/64878899#64878899

Since we're stuck with a version-pin of Qt 5.12.1 until a new release resolves this unrelated bug affecting Debian, it would be a good idea to set the QT_MAC_WANTS_LAYER=1 environment variable at runtime so that Gridsync can function properly for Big Sur users.