This is about a rules bug. I did not find any other posts on this specific topic.
The above image shows the feedback from a fighter trying to attack a goblin when they are both inside a fog cloud. Attacks involving a party inside a fog cloud never have either advantage or disadvantage unless one of the parties can see through the fog cloud in some way. Targets inside a fog cloud are heavily obscured -- imposing disadvantage -- but are also blinded, which grants advantage. Any amount of advantage cancels out any amount of disadvantage. The same applies to attackers inside a fog cloud: their targets are heavily obscured to them because of the fog in the way, but neither can they see the attack coming.*
*: The wording of the vision errata is sometimes taken to mean that a creature inside a fog cloud can see creatures outside it just fine. While I think that interpretation is nonsense, I'm deliberately ignoring that scenario for purposes of this bug report.