This is called font-linking. mIRC will do its best to display characters using the font that you have chosen, however if the font (especially an old non-unicode font like FixedSys) cannot display certain characters, mIRC will ask Windows for a font that can optimally display them and then alternates between fonts where necessary when rendering the text. It will do its best to revert back to your chosen font when possible. However, the method for how this is handled depends on the feature: mIRC's text display has its own method, described above. Windows editboxes/controls have different levels of support for this, depending on the type of control, its age, richedit DLL version, and so on. So the behaviour can also vary across different versions of Windows. Support for font-linking has been in mIRC for a decade or two. There have been no changes to how this works in recent versions of mIRC.
