Thank you for chiming in!
The first two options didn't work because InDesign has no option in its menus to arrange the app window itself, just its components. Also, the menu bar of the app within the macOS interface was always visible. The top portion of the app's window had disappeared beyond the display without, at the same time, appearing on the other display.
I used Magnet in the past but stopped some time ago.
What worked was to move the pointer to the lower edge of the window until the pointer became a double-edged arrow. At that point, I held down Option (Alt) and dragged up. This resized the whole window vertically in both directions. A double-click on the window's title bar then reset its size to maximise. In case also the bottom were out of bounds, I could have added Shift and tried from one of the side edges.
Still, why Adobe should feel exempt from adapting to the OS they are being hosted upon is beyond my perhaps limited understanding.