Okay from looking around, Qt seems to put its cache files everyplace even though we provide it with a local storage path.
This is so strange ...
It seems that some caching is done in our Sigil Prefs inside the designated local-storage folder.
It seems a QtWebEngine folder is also created there but it is basically empty always.
Then I found some recent cache files in ~/Libary/Caches/sigil-ebook/sigil/ locations which is Qt's StandardPaths::CacheLocation
Here is what Qt's StandardPaths::CacheLocation that defaults to on all three platforms:
MacOS: ~/Library/Caches/<APPNAME> (APPNAME for me was sigil-ebook/sigil/)
Windows: "C:/Users/<USER>AppData/Local/<APPNAME>/cache"
Linux: ~/.cache/<APPNAME>
If I look inside that I do find a cache called "Preview" as expected and it does have some data.
But there is a bunch of stuff in our Sigil Prefs folder in the assigned "local-storage" folder that is updated when working in Sigil as well.
I have no idea which of these locations on Windows, that the second (or third or ...) instance of Sigil is being locked out of.
I have no idea why QtWebEngine is spreading its working files all over the place.
So I am not sure which name or path needs to be extended by a uuid.
I tried wiping out all of those cache locations and local-storage locations and restarting Sigil and they are all being recreated and used.
What are people seeing under Linux and Windows?