“Now where’s the halacha that says that “covered in any position” is the appropriate measure?”
Very strange question. That’s like asking “Now where’s the halacha that says that “all ingredients are kosher” is the appropriate determination of kashrus?” For something to be kosher, all the ingredients must be kosher. You can’t say, well, 11 out of the 12 ingredients are kosher so we’ll certify the product as kosher.
If something has to be covered, obviously it must be covered at all times. Why would there be an exception for when its a hot day or a windy day or walking up a staircase or sitting down? If it can be uncovered when sitting down why must it be covered when standing or walking? If something has to be covered, obviously it means always covered, not covered 85% of the time.