The navigator view can easily have hundreds of items for larger documents, with lots of clicking It was be useful if one had a text box to use for filtering these items - like Thunderbird's folder quick-filter box, for example. To conserve vertical space, there should be a toggle for the filter box, e.g. a button, and/or perhaps if you started typing while a navigator element was focused.
Oh, I guess another example would be the filter box we have in the Cross-Referencing dialog.
*** This bug has been marked as a duplicate of bug 43511 ***