Maximize the main DigiKam window with either thumbnails or preview. Click Full Screen icon to switch to full screen mode. Click Exit Full Screen icon to exit full screen mode. Window usually restores to "normal" mode instead of maximized mode. I have found that this failure does not always happen, but typically once it fails, it continues to fail.
This problem still reproducible with current 5.8.0 - pre-release Windows bundle available here : https://files.kde.org/digikam/ Gilles Caulier
I don't see this problem with the 7-Dec build of Win 64. Exit Full Screen returns to previous Window mode, whether that be maximised or normal.