Sub-window hidden behind another window... any way to retrieve other than minimizing?
In current versions of macOS (the last few, in fact) allow you to tap the F3 key to show all windows of the current application. On MacBook built-in keyboards and Apple's own keyboards the F3 key has an odd looking icon that represents this. Similar to the F4 grid of squares that brings up Launch Pad.

Windows will shrink in size so they can appear all together on screen allowing you to tap on the window you want to restore the App with that window frontmost.