1. From Thumbnails mode, double-click an image to open it in a bigger size;
2. It enabled Preview (F3) mode, image is now bigger, but there's a row of thumbnails on the top.

To hide it, I have tried:
1. Moving lower border of this row, it can change height of thumbnails row, but there's hard lower limit on this;
2. RMB click on border or anywhere in this row, there's either scroll bar menu appears, or nothing at all;
3. Check amongst square buttons in the top left corner, where you can rotate image and show full tags, there's nothing related to this row;
4. Check View main menu (Alt+V), still don't see how to hide row of thumbnails.

This is a lot of effort trying to hide this UI element.

Fullscreen mode (Ctrl+Shift+F) hides it for some reason, but also changes something else.