What does 'come across' mean?
After dressing for dinner, he walked across to the Senior Common Room for sherry.
...
Jacko came in and walked straight across to him.

Question

What does 'come across' mean?
After dressing for dinner, he walked across to the Senior Common Room for sherry. 
...
Jacko came in and walked straight across to him.

Answer

The word here is "across". Now this word is in every dictionary.

Answer

Jacko walked straight to him.    Jacko walked directly to him. 
If you  look 'across' the room, you look to the other side of the room.  This line of sight or vision is the path or route that Jacko walked.

Answer

come across sth means find sth by chance
it has nothing to do with walk across the room