In the dictionary
Across: from one side to the other side of something
Side: [countable, usually singular] either of the two halves of a surface, an object or an area that is divided by an imaginary central line
Have a look at this paragraph from a story written by a native English speaker.
Ben was about to respond when suddenly his stool flew out from under him and across the room. Ben looked at his stool in surprise. It was halfway across the room.
Based on the 2 definitions above, this is what I think as shown in the below picture. The room is divided by an imaginary central line. Ben is on 1 side of the room. Then his stool under him flies to the other side of the room.
But I don't understand "halfway across the room"
And these are the pictures from the story
What does "halfway across the room" mean?