There can be several reasons why PopChar might fail to detect which font is being used in the active application:
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If font detection does not work at all in a particular application, this is very likely because the application does not have a font menu in the menu bar. If font detection generally works in an application but fails in some situations, a dialog window may be in the front or the font menu may be temporarily unavailable. In this case, PopChar will display the font that has recently been used in this application. If you need a different font, temporarily select this font and return to the "As in Document" setting afterwards.
In general: If automatic font detection fails in a particular application, you need to manually select the desired font in the font pop-up menu of PopChar. PopChar will then remember the selected font for this application.