No, Starbucks shares are not down 50% today
Except they're not.
On Thursday, Starbucks will begin trading on a split-adjusted basis after its 2-for-1 stock split was paid on Wednesday after the market close.
This means that for every 1 share of Starbucks you owned, you now have two at half the price. You can read the company's announcement from March about this here.
Everything else is the same. (Except earnings per share, which are exactly half what they were before.)
In pre-market trade on Thursday, shares of Starbucks were up about 0.6% to $47.90 per share.