This marks the one and only time I will describe how to create environment variables in Windows 10/11. Bookmark this post for future reference.
1) type 'env' (no quotes) in the taskbar search area
2) click the resulting 'Edit system environment variables' link
3) click the Environment Variables button at the bottom of the open Advanced tab
4) in the User variables for <user> section, click the 'New' button.
5) in the 'Variable name:' field, type SIGIL_ALLOW_CODEVIEW_DROP
6) in the 'Variable value:' field, type '1' (no quotes)
7) click the OK button
8) click the OK button
9) click the OK button
10) Launch Sigil and verify dropping in Code View works.
To disable it again:
1) repeat steps 1-3 above
2) highlight SIGIL_ALLOW_CODEVIEW_DROP in the top window and click the 'Delete' button
3) click OK
4) click OK
5) launch Sigil and verify dropping text in Code View no longer works.