[UPDATE:]
The feature I was looking for does not exist for image thumbnails. I was remembering a feature for the Hover Box called Hover Box Status, though "status" does not really describe the information available through the feature. There is no similar feature for thumbnail overlays. On thumbnails, you can display dimensions but no other information (AFAIK).
For my own reference, and in case anyone else is looking for this feature, it is described here: https://www.xyplorer.com/release_19.40.php. My own short description follows.
While XYplorer is displaying a Hover Box, you can adjust the amount of information shown along with the file preview by pressing "S" on the keyboard. Pressing "S" while XYplorer displays the Hover Box cycles through the following: file preview only, file preview + file name, file preview + file name + file details.
Toggle the file icon in the Hover Box by pressing "I" (capital i, not L) on the keyboard while the Hover Box is displayed.
Change the background color between dark and light by pressing "B" on the keyboard while the Hover Box is displayed.
Change the size of the Hover Box by pressing add or subtract on the number pad while the Hover Box is displayed.
Cycle transparency through neutral, grid, white, black by pressing "T" on the keyboard while the Hover Box is displayed. This feature is detailed here: https://www.xyplorer.com/release_19.90.php
Thanks.
XYplorer Beta Club