This is something that would need to be addressed within Froala itself or you may need to look at whether there are "hacks" to workaround the behavior (such as limiting the max height of these dropdowns). It's not a context that we use within XF so I don't think it can really be considered a bug on our end.
Froala may consider it as some sort of enhancement, though I'm not sure they will consider this as a particular use case that they are intending to cover.