In my code, I provide a tool to increase or decrease font size by a small proportion.

I create a string containing the font family name, and the resulting size rounded to 2 decimal places, then use that in gtk_font_button_set_font_name().

The result is a font button displaying 4 decimal places, which I'm sure the user does not need to see.

I'm guessing that the string is parsed, then the size is converted to PANGO_SCALE units, then for the button converted back to decimal without appropriate rounding.

e.g. I use "Sans 12.15" as the argument to set_font_name, but the button has 12.1504