As a non-resident alien if the only business you have in the U.S. is in investments (stocks, mutual funds, commodities, etc.) within a U.S. dollar-denominated brokerage firm or other agent, you are subject to the following tax guidelines. In terms of capital gains, non-resident aliens are subject to no U.S. capital gains tax, and no money will be withheld by the brokerage firm. In terms of dividends, non-resident aliens face a dividend tax rate of 30% on dividends paid out by U.S. companies.This 30% rate can also be lower depending on the treaty between your home country and the U.S., so it is important that you contact your brokerage firm to verify the rate.

Your statement "a few years less than 163 days" I presume you meant to say a few days less than 163 days. The guideline is actually 183 days, The rule is non-U.S. citizens who hold green cards and have been in the U.S. for at least 31 days during the current year and more than 183 days in the past three years are classified as resident aliens for tax purposes and are subject to different guidelines than non-resident aliens.

To be considered a non-resident alien, a person must meet several guidelines. First of all, the person cannot have had a green card at any time during the relevant tax reporting period (e.g 2016) and cannot have resided in the U.S. for more than 183 days in the past three years, including the current reporting period.