Safari Always Opens New Windows (⌘N in Tabs

This issue is about manually trying to open a new browser window, NOT a website creating a new window.

Since updating to Safari 12, trying to open a new browser window with the keyboard shortcut (⌘N) causes a new tab to open in the current window. (None of the options in the Tabs tab in Safari's preferences changes this behavior, and I have tried deleting the preference file.) Under the file menu, the shortcut for opening a new window is ⌥⌘N.

This is not an issue on my desktop machine. I am running Mojave and wonder whether there is a new Safari or System Preference setting that might be causing this.