Each time this has happened when I've been using a program which uses the X FV() function, to plot lines on the screen, though maybe it has also happened by me clumsily pressing a key combination in the area of the big red STOP key.
It seems rather "deliberate" due to the neat line at the bottom, and the fact I can still list programs within this 36x4 box. But I am no longer to address areas of the screen outside the box.
Does anyone know what the cause of this might be? Is it hardware (the monitor is generally a bit shaky and sometimes fails to show anything but a white horizontal line in the middle of the screen), or is it due to some screen addressing error (though in general the Focal interpreter does not seem to complain when you try to draw a line which goes off the screen), or is it some arcane key combination I accidentally pressed?
I'm using an Elektronika BK 0010-10 with the Blok MSTD 3.069.007 Focal Cartridge, and a black-and-white MS 6105.1 monitor.