Here’s something you can try:
open the file
%CONDA_PREFIX%\Lib\site-packages\mfixgui\main.py
in a text editor.
Change the commented-out lines (around line 88) to the following (uncommented):
QtCore.QCoreApplication.setAttribute(Qt.AA_EnableHighDpiScaling, True)
QtCore.QCoreApplication.setAttribute(Qt.AA_UseHighDpiPixmaps, True)
This cleared up the issue for me. (It also makes the GUI larger on the external monitor but I think that’s an improvement. You can reduce the font size in “Settings” if it’s too big for you)
Please let me know if this clears up the problem. I will probably add this to the next release, or make it an option users can select in Settings.