[KevinH]

There are Windows and macOS plist settings to prevent auto "cleanup" of /tmp files by your OS. You will need to do some googling to find out how to set them.

Or update to a recent version of Sigil. The current version will not use OS tmp folder at alll by default. Older recent versions of Sigil will allow you to define your own tmp folder replacement.

Sigil 0.9.14 uses the older Qt Webkit which is now a security nightmare. It has had no security bug fixes or updates in over 5 years. I strongly suggest updating Sigil and adding PageEdit if you often work in the old BookView.

Here is a link describing some of the settings files on macOS that control this "auto cleaning stupidity".