Okay I repeated those tests but this time with the second monitor set on the left of the primary monitor, and it all worked exactly as expected on macOS.
So I can not recreate what you are seeing. It is most probably Windows specific or specific to your setup somehow.
So I have posted a debug Windows build here:
https://github.com/kevinhendricks/Bu...OnMac/releases
Please grab:
Sigil-master-screen-debug-Win-Setup.exe
To use it you need to set the SIGIL_DEBUG_LOGFILE environment variable to point to a text file on your Desktop (or someplace you can easily access).
Then run the working setup with the secondary screen on the right.
Then copy the resulting debug log file to a new file called "working_log.txt" and empty it in prep for the version that crashes (or just change the name of the file in your SIGIL_DEBUG_LOGFILE environment variable).
Then run the version with the second screen on the left, that causes the crash.
Copy the rename the resulting debug log file to "crash_log.txt" then please zip up and post "crash_log.txt" and "working_log.txt" here so that we can see it and look to see if anything obviously wrong is being detected.
Thanks!