What passes as a "single topic?" Often, "additions" are included in these last-second, "must sign" bills because they face resistance and are unlikely to pass

What passes as a "single topic?" Often, "additions" are included in these last-second, "must sign" bills because they face resistance and are unlikely to pass on their own. No representative, nor the president want to be responsible for a "government shutdown" so anything that makes it into the last version of the bill is likely to become law.