Roku dipped about 6.5 percent Tuesday morning after Citron Research called the stock "uninvestable" in a tweet in light of changes to the competitive landscape. It recovered slightly in the afternoon, closing down 3.1 percent after Citron clarified, "we are watching $ROKU from the side."

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The stock surged 25 percent the day before after releasing early Q4 viewership numbers that beat analyst expectations. Roku, which offers streaming players and TV models that allow customers to stream content from various services, also announced Monday that Westinghouse Electronics would license Roku's TV software. Even Citron initially was positive on the news, saying in a tweet that it had originally gone long on Roku at $35 per share.

But on Tuesday, Citron said in the tweet it has "to recognize when the story has changed. APPLE TEAMING UP WITH SAMSUNG., ROKU CEO selling last week, and short interest at lows. Risk/reward no longer there. Expect big retracement."

Apple and Samsung, typically fierce rivals, surprised the public by announcing a deal on Sunday to offer Apple services on Samsung devices. As part of the deal, some Samsung smart TVs will be able to access movie and TV content through iTunes, giving customers another streaming alternative to Roku. Customers will also be able to send content to their TV screens through AirPlay 2 on iPhone and iPad, according to the announcement Sunday.

Roku's stock was still up more than 21.2 percent for the week as of Tuesday's close, with a market value of about $4.5 billion. The company is expected to report its fourth quarter 2018 and full year earnings in February.

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