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I recently learned how to build and use a custom font thanks to the documentation page: https://www.klayout.de/doc/about/basic_lib.html#h2-44
I created a font called "MY_FONT" (in file "my_font.gds") and I used it successfully in a test file ("font_test.gds"). This means that I could insert a PCell, choose my custom font from the menu, and see that the graphic display is indeed using MY_FONT, because it is clearly different from the standard font. This worked as expected.
This looks like a bug, can you test or tell what it wrong?
Olivier (Klayout version 0.25.3)