None of this inline-logic-inside-JSX stuff please... it hurts readability, hides control flow, and increases indentation

Should this be wrapped in React.memo?

Let's void .then syntax and always use await / async. If this breaks types, update them to accept any function that returns mixed (doesn't have to specifically return void)

Why are we passing our vertical bounds? Does the FullScreenThreadMediaGallery have the same vertical bounds? How are we going to maintain that property going forward? Should FullScreenThreadMediaGalleryRouteName just measure its own vertical bounds, since that's what we need for the ImageModal animation?

It's a little unusual but fine imo. Only thing is that you don't need React.useCallback... you never pass the callback itself anywhere, and so there's no reason for it to be memoized

On the other hand, THIS needs to be memoized

We don't need to regenerate this function whenever offset changes. We only need to regenerate it if offset becomes undefined or stops being undefined. Can you update the dep list to reflect that, and reduce the amount of times onEndReached needs to be regenerated?

We need to coordinate with the design team if we're adding a color – can you send the UX team a message about this?

This literally does nothing

We shouldn't be memoizing the whole thing here