don't repeat the sender/avatar when it's the same for two consecutive messages
then also skip the context for threaded messages, when it's the same for two consecutive messages. to still visually distinguish threaded messages from normal messages, give them an alternative background
note that the latest version of the patch fixes the ugly 1px or 2px hidpi gaps by adjusting the full rect by
-1, -1 also in the x1/y1 direction.