[KevinH]

You still have broken tabbing with the system trying to convert the extra window into a tab. Please take a snapshot of the menu that is produced under the View menu in Sigil right now and post it here.

Just in case this is cache related. Here is an even more extensive set of instructions for cleaning up.

1. Exit from Sigil completely, verify this with Activity Monitor or ps
2. In System Preferences->Dock make sure prefer tabs to Windows is set to manually.
3. remove the Sigil ini file
4. remove any and all things related to Sigil in ~/Library/Caches/

// New step just inserted!
5. remove any and all things related to Sigil in ~/Library/"Saved Application State"

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6.remove any and all things related to Sigil in ~/Library/Preferences

7. restart your computer making sure the box to reopen windows is NOT checked.
8. after restart, start up Sigil but do NOT use any key sequences
9. verify if the extra title bar still exists. It should not. If so look at the View menu and make sure the Show Tabs menu item is not activated. If activated, toggle it off.
10. once off and the extra title bar has gone away, resize the Sigil windows and then properly exit Sigil to get a viable ini file created.

If after this sequence, you still see two nested title bars, then your system is either hosed or you have third party software that is either tabbifying windows or adding extra decorations.

To test if something in your account is interfering with Sigil, create a new user account and install Sigil from a fresh release download into that account onto its Desktop and run Sigil from your Desktop *after* making sure that Dock preference setting is set to Manually for this new user account and test Sigil that way.

Other than that, your only viable workaround is to use Sigil 0.9.10 until the next release comes out and then test again.