EmEditor Professional (64-bit)
Version 16.3.0 beta 6
Opened 10K line TXT file with CSV data.
In Comma Separated CSV mode with Headings 0.
Select a cell at the top and it gets the default light green (#CAFFCA) coloring with a darker green (#008000) border.
Drag scroll bar down (leaving cell selected near top) to about line 1600 and the column with the selected cell starts getting the darker green border.
Mouse scrolling upwards shows that the borders stop getting drawn at about line 1560.
Selecting a cell on line 5000. Now it draws the borders after scrolling up 1530 lines (to line 3670) and after scrolling down 1560 lines (to line 6561).
You might have to mouse-wheel down or up a bit to get it to show up.