If you still haven't clicked "Post" or "Preview", and the browser crashed/PC shut down unexpectedly, there will be no draft for that particular post.
Drafts will be saved after you've clicked either of the two buttons.
And another backup of the final version after clicking "post".
Exactly this should be changed to save drafts as only we start to write a new post because I already lost a couple of times very valuable post, only because I haven't click on the "preview" button and it was to soon to publish and unfinished work and to click "post". Computer crash, lost internet connection or just a misclick is enough to lose the post completely if you haven't previewed it soon enough.
Of course, I learned already my lesson and now if I know or see that the post will be bigger than average one, I click on "preview" and save the draft just after I start writing, but to be honest if you don't need to use special characters, then this is not needed and that is why so many times is overlooked by many.