Their argument was along the lines of "The requirements and design don't specify what should happen if you move and delete at the same time so it can't be a bug. Behavior that doesn't violate the design but also doesn't lead to the result the user wanted is a user error". My argument was that we can't always specify the interaction between arbitrary features other than "If the user does two things at once, at least one of them should be executed, ideally both" and "the program shouldn't crash just because the user did something unexpected". Otherwise our design document would be ten times as long.