Recently the search box in the corner began elongating itself to take up about half of the page width (approximately out to where the username would be). This elongation takes place after typing the first character of a search query. The shift would be more expected if it happened upon clicking in the box rather than only after starting to type, but either way it's a little confusing to the eyes. What is the purpose of this feature?
Generally this is to encourage people using more words in search. Larger box, more words, etc. If the box was made very large by default, it would take up so much room that it would displace other necessary elements in prime real estate. Hence, expanding "on demand".
See other services that do this in my answer here: