What is this magnification box in :LR Classic

It looks very much like the [Guided Upright Tool] in the Transform panel  available by clicking the hash icon, or keyboard [Shift+T]

However your screen-clip indidates the tool is not active as the hash icon is visible (the icon goes to a grey dot  when active) and yet you continue to see the square box on screen.

There may be a glitch (very technical term) in your graphics card, so things to try might include-

A restart of LrC.  A computer restart. An update for your graphics driver. A reset of the Preferences file. Or even a re-install of LrC.