It is a fairly common GUI pattern to have one or more controls that are relevant only if a particular checkbox or radio button is selected, and to indicate this by indenting the dependent controls. For example, in Gnome's Window Preferences:

    [/] Select windows when the mouse moves over them
        [ ] Raise selected windows after an interval

In Orca's "Key Echo" preferences:

    [ ] Enable key echo
        :/: Enable alphanumeric and punctuation keys
        :/: Enable modifier keys
        ...

    [ ] Enable echo by word

And in Gnome's Network Proxy preferences:

    (*) Automatic proxy configuration
        Autoconfiguration URL: [___________________________]

Ideally, the left edge of the dependent control(s) should be exactly aligned with the left edge of the *label* of the checkbox/radiobutton/expander. In other words, the dependent controls should be indented by the width of the checkbox/radiobutton/expander, plus the width of the padding between that control and its label. Unfortunately none of the above examples get it exactly right, because the control and label widths differ by theme, and there is no easy theme-independent way to do the indentation (short of asking for indicator-size + indicator-padding and listening for theme changes, which would be excessive).

This could be fixed by adding an "indentation-level" property which can be applied either to boxes or to individual controls For sanity, it should assume that checkboxes and radio buttons have the same width and padding (perhaps by using the maximum of the two). Expanders are often wider, but exact indentation for those isn't as important.

[Thanks to Johan Dahlin for providing the technical details.]