But what to do with such a person if you can't/won't ban them from posting? Delete his comments? Don't engage until he tires himself out?
How about ignoring details and only focus about the technicality?
Looking at that way too long thread, both sides seem to be blamed on way or another. From what I can see, HDR support missing is actually an issue. Instead of letting him know they accept the bug, they are feeding him with rhetorical questions .
Developers should not worry about this people. They should not take pride in making people happy but take it from knowing millions and millions of people will benefit from your code. No matter how much of an asshole the bug reporter is, a bug / missing feature is just that.
Then the OP started complaining... that it's taking too long to fix, that ffmpeg developers are not paying attention to his bug... etc, all while ignoring everything said to him
That person is causing all the pointless animosity.
I think your assessment that developers should not derive any pride from happy users and are merely there to serve a greater good is a bit far from reality. Maybe they should not worry about things like this but then again these are also just people living their lives. This is not a corporate entity where you engage with a PR representative, these are people, doing what they like to do.
If you take away the voices of such people, pretty much the only voice left speaking is the voice of corporate money, which is usually anthithetical to the principles of open source development.
EDIT: Especially considering features like Gopher and MSP were deemed worthy of attention, the fact that his point about HDR metadata being silently stripped was not being addressed in a year is concerning.
Being an asshole is bad, specially when you are asking for help. Not being an asshole doesn't make you a corporate shill nor being an asshole make your feedback "better".
This is doubled by the fact that a script was available as workaround...
Also, I see it eventually got some support in spite of discussion so kudos to ffmpeg devs.