It is not just Preview that is not being shown but instead the full main wiindow that is not showing. So disabling the gpu acceleration would not help you to see Sigil's MainWindiw. Changing the backside graphics kit may help to opengl may help. But that would require setting the relevant RHI env variable.
Here is what Qt's docs say about there RHI environment variable:
Quote:
QSG_RHI_BACKEND
possible values: vulkan, metal, opengl, d3d11, d3d12
Requests the specific RHI backend. By default the targeted graphics API is chosen based on the platform, unless overridden by this variable.
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So you could try setting a QSG_RHI_BACKEND environment variable to "opengl" or try "d3d11" or try "d3d12" and restarting Sigil after each to see if which if any of those 3 possible setting help.