Learn Swedish: Att de ständigt bråkar! - I can't believe that they are constantly fighting!

This is an independent att-clause used as an exclamation to express frustration, shock, or indignation. You can think of it as a shortened sentence where the main clause (like 'I can't believe...') has been dropped for emotional effect.

Why is the word order de ständigt bråkar instead of de bråkar ständigt?

Because this exclamation is still grammatically a subordinate clause (a sub-clause). It follows the BIFF rule, which means sentence adverbs like ständigt (constantly) must be placed before the conjugated verb (bråkar). If this were a standard main clause, the order would be De bråkar ständigt.

Could I use other adverbs in that exact same position?

Yes! Any sentence adverb will go before the verb in this structure. For example, you could say Att de aldrig bråkar! (I can't believe they never argue!) or Att de inte bråkar! (I am surprised they aren't arguing!).

Does bråkar imply they are physically fighting?

Not necessarily. While bråkar can mean physical fighting (especially among children), for adults it usually means loud arguing, bickering, or making a fuss. If you wanted to explicitly describe physical violence, you would use the verb slåss.