looking for a preview window plugin

[Ofnuts]

Windows ➤ Dockable dialog ➤ Navigation window, then either you have some way of keeping it above everything (fairly straightforward on my Linux, but you may have Window utilities for this), but also since it is "dockable" you can add it anywhere in your Gimp window, so it's always in sight,  but then its size depends on the size of the dialog it's docked in. To add it to an existing dock, you can also use the Add tab menu of the doc (in the menu).