A US trade body has opened a patent investigation into three of Taiwan's biggest tech makers, an unwelcome headline for shareholders just as Taiwan's market faces a rough reopening.

• The US International Trade Commission launched a Section 337 probe over power-system patents claimed by American firm Vicor, naming Foxconn, Delta Electronics and Quanta.
• Foxconn rushed out a filing saying nothing has been decided yet and a legal team is already working on it; its subsidiary Industrial Fulian says the products are still in internal testing, so no hit to sales.
• Delta says its products and operations are unaffected and it will not comment further now that lawyers are involved.
• Taiwan's trade office calls it a company-to-company patent fight and says these cases usually take 12 to 16 months to resolve.
• The timing stings: US chip stocks slid over Taiwan's holiday break, dragging night-session futures down sharply and setting up a heavy catch-up drop when trading resumes.

Outlook: Expect a volatile Taiwan open and no real clarity on the patent case for a year or more.