Resident alien (visa holder) that may leave the US in a few years. How will my US stocks and tax be calculated at that time?

I buy some stocks on Fidelity and I may leave the US in a few years. I wonder how will it be taxed when I leave the US? I have two scenarios:

• Sell the stocks at the price near the time I leave. This creates no complications.
• Still keep the stocks and sell it later, maybe online or when I come back on a visitor visa.

I am aware that Fidelity won't allow buying mutual funds after leaving but that doesn't matter, I am unlikely to continue to invest after that. I wonder what will happen in the second case? How will I pay taxes on that?

Additional info: my country does not have a Tax Treaty with the US, but they also don't tax capital gains from stocks (for now).

TIA