Strange bug after switching from another app

Sigil 0.9.8 with OS X 10.11.6

1. Sigil Document open, text window frontmost, cursor anywhere on page.
2. Switch to any other application.
3. Switch back to Sigil by any method.
4. A capital S is inserted at the cursor every time. Sometimes the cursor moves down one or more lines before the S is inserted.
5. The problem continues after multiple restarts and shutdowns.

I have tested many applications. This ‘S’ problem does not occur.
I am using Desktops (Spaces—I wish Apple would use just one term.). I don’t know if that matters. I know how to turn Desktops off, but I don’t want to lose all my application settings.

Has anyone else seen this? If not, then I’ll assume that parts of my OS are corrupt.