Preferences window can't be closed via keyboard shortcut
After opening the Preferences window, there is no way to close it via keyboard shortcut.
Things I've tried: ESC, ctrl+W, ctrl+F4, alt+F4, shift+ctrl+W ("Close Tab" shortcut key), shift+ctrl+Q ("Close Window" shortcut key).
In addition, if the Preferences window is opened via a keyboard shortcut (e.g. it might be set to "alt+,") then the keyboard focus does not switch to the Preferences window. NOTE: if the Preferences window is opened via the mouse menu, then the Preferences window DOES receive keyboard focus.
When the above two issues are combined, a 3rd usability issue arises: If the Preferences window is opened via keyboard shortcut, and the user then presses the "Close Tab" or "Close Window" shortcut keys (i.e. expecting to close the Preferences window), then the user unexpectedly ends up closing an open shell.