Keep two counts: working copy and saved file. Opening copies the saved drawing into working memory. An edit changes only the working copy. A successful save replaces the saved drawing with the working copy. Discarding unsaved edits and reopening restores the saved drawing. Autosave and recovery are off in this paper model.
Worked example
Open a saved drawing of 3 squares; add 2 and save. Both copies now have 5. Remove 1 without saving: working has 4, saved has 5. Discard that edit and reopen: the drawing has 5 squares.
A Different Drawing
The saved drawing starts with 10 dots. Open it, add 3 dots, then successfully save to the same file. Next remove 5 dots without saving. Finally discard the unsaved edit and reopen that saved file. All steps use one fictional file; autosave and recovery are off. Do not delete files or switch off equipment to test this story.