[DiapDealer]

I think we might be closer. There are no infinite loops and multiple instances work.

The named storage is created in Sigils prefs directory (under the 'cache' folder) and they are cleaned up properly after Sigil exits.

The only problem is that the named storage is being duplicated in a C:\Users\<??????>\AppData\Roaming\sigil-ebook\sigil\QtWebEngine (not the proper Sigil prefs location) and they are NOT being cleaned up after Sigil exits. They are being orphaned. There's never any content in the orphaned folders. Even when Sigil is still open. It's like they're being created there before the code that properly creates the caches in Sigil's prefs directory takes effect or something. Very weird.

I think it might be the very act of trying to change these default locations that's introducing these anomalies on Windows.

We'll have to revisit tomorrow sometime.

EDIT: Sorry, I forgot to attach the debug output. It's reporting the correct paths (not the stuff in Roaming) by the way.