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HDRP
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I don't know how much of the documentation is usually pulled to tooltips. The problem I see is the maintenance, i.e. if options are removed/added, then in addition to the docs the tooltips needs to be updated as well to reflect the change, which I guess is easy to miss. How is this with other dropdown menus?

The best answer I can give is, it depends :). Some dropdown tooltips are brief when it's self explanatory and some are way longer.

I personally prefer having a longer tooltip as soon as I consider there can be confusion or questioning for the user as to what these select options do. And it saves lots of time searching for details in the documentation.

As for your concerns about maintenance, I would say, @Unity-Technologies/gfx-qa-hdrp is already very sensitive about tooltips. HDRP is already not easy to learn, better take all our chances :)

Here's a few example from materials and overrides:

Double Sided GI

Global Illumination

Reflections Tracing mode

Sure no problem to add it if it's already a policy to do so. Ideally the editor would pull the tooltips from the doc files instead of having them embedded in the code,