Disable tabbing completely. The user defaults hack isn't sufficient because...

Disable tabbing completely. The user defaults hack isn't sufficient because holding down option causes a new window to open as a tab. Issue 6197

sources/iTermWindowImpl.m

@@ -381,6 +381,10 @@ ITERM_WEAKLY_REFERENCEABLE
     return [super makeFirstResponder:responder];
 }
+- (NSWindowTabbingMode)tabbingMode {
+    return NSWindowTabbingModeDisallowed;
+}
+
 NS_ASSUME_NONNULL_END
 @end