How to delete a file saved on levels

[Ofnuts]

They are visible on my laptop screen (1920x1080, 142ppi) and my secondary (2560x1440, 27", 109PPI).

Do you see these handles in your Histogram dialog (Windows ➤ Dockable dialogs ➤ Histogram):

   

There are also two such handles in the Threshold tool.

I don't think moving to 2.8 will change anything. it is even less hi-def-aware than 2.10.