[KevinH]

Okay, I forced it to use tabs instead of normal windows using the System Preferences -> Dock settings and sure enough, macOS tries to inject a tab bar right below the title bar and that messes Sigil up badly as it forces all selections in BookBrowser to be off by one line. This also messes up the ini file when saved in that sad state.

I strongly recommend always setting that Dock Preference to "Manually" and then removing the keyboard shortcut Apple(Cmd) Shift T from the shortcuts used by the macOS system.

Apple should not try and tabify an already tabbed multi-frame QDockWidget ever!

I wonder if there is a way to disable this stupidity using something in the Info.plist of the Sigil app.

Only Apple would allow itself to inject code into a thrid party app.