I am using DirOpus v13.18 x64
DirOpus renders truncated file names strangely and disregards the chosen font for part of the name. See video:
What is the chosen font?
Monaspace Argon. Buit it looks like the segment that get truncated is being rendered in the default font.
I was able to reproduce it, it seems to be something the font does itself when the area it's told to render into is smaller than the size it needs. Instead of clipping the text as it normally would it reduces the font size (or maybe it is dropping back to another font as you suggested, it's hard to tell).
Might be something to do with the "texture healing" feature they talk about, though I'm not sure.
The "static" and "frozen" versions of their fonts don't seem to do it, just the "variable" one, so I advise switching to one of those.
I was using the 'static' version of the font, not the variable one. I switched to 'Monaspace Neno Var" and the problem went away--so it does appear to be an issue with the static font.
Thanks for helping to figure it out.