Are you by chance setting Windows to run Sigil in some Compatibility mode? Or what about using a dark theme that might confuse Qt?
On searching the web for "Windows app open to white screen", I found the following link for a different Qt app:
https://community.atlassian.com/t5/H...w/qaq-p/277535
Which says:
Quote:
What also seems to work:
Add a new user environment variable named "QT_OPENGL" with the value "software" and reboot/relogin
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Perhaps the same thing will work for Sigil since it is a Qt app?