To be honest, I'm not sure if cascading is the right term, and if it's not then this can just be edited in the future for clarity.
I need a taxonomy term widget that is a drop-down list, and all that is initially populated are the top-level taxonomy terms in the particular vocabulary associated with the widget. After selecting a top-level term, another drop-down list is added next to the previous (the one you just selected from) with all of the children of the selected term. The parent drop-down list is then disabled.
You should be able to click a link to go "up a level", basically removing the right-most drop-down list at the time and re-enabling the right-most list after that.
For example, say I have the following structure:
The initial drop-down list would just contain: A, B, and C. You now select 'A'. The list with A, B, and C would then become disabled, and another list would be added directly to the right of it with the terms: AA, AB, and AC.
The term submitted along with the form would be the term represented by the right-most disabled list's selection at the time.
Is there anything that exists with this functionality (or similar)? If so, I haven't seen it (and I'm looking hard). Otherwise I guess I'll just have to code it up myself, but I'm hoping I don't have to invent a new wheel.