Alternative Design for two nested Tabbook Widgets in one dialog (Screenshot inside)

Answer 1

Nested tabs are not only ugly, they're confusing for the eye. It's hard to see at a glance how things are organized.

If the total number of panels is ~20 or less, consider flattening your hierarchy and making it all one level, either as horizontal tabs, or, if that doesn't fit in a single row, vertical tabs or a list box. The hierarchy remains represented by the order and names of the tabs / list items (e.g., Interpreter Error Handling, Interpreter Format, Console Colors, Console Font, Console Counsel).

This has the added benefit of allowing the user to get to any panel in a single click. It also makes it easier for users to understand the organization of content. Higher level names tend to be vague (e.g., your "PHP Options" tab above), making it hard for the user to judge if it's what they want or not. By exposing the detail-level names, the users have a better chance of spotting the panel they want, and it helps clarify what high-level name means.

Another option is to move the hierarchy up a level and have separate dialogs for each top-level tab, each accessed by a different pulldown menu item. Who said that all preferences/settings have to be in a single window? This is preferred if the task typically involves the user going to a single top-level panel rather than moving among multiple panels.

And fix that "Okay" button caption, while you're at it :-).