Opponent has a 6 might Unit in their battlefield. I played Hextech Ray there. Now I move with a Unit to this battlefield and after passing Focus, opponent plays Rebuke on my Unit. Do I have after resolve a window to cast another Hextech Ray to kill the Unit. Asking because of Action, not reaction speed of Hextech Ray.

Yes, you do have a window to cast another Hextech Ray after Rebuke resolves.

Here is the breakdown of why this works, based on the rules for Showdowns and the Chain:

The Sequence

1. Showdown State: When you move your unit to the battlefield occupied by your opponent's unit, a Showdown begins. The state is Open (no chain exists).
2. Opponent's Action: Your opponent plays Rebuke (an [Action] spell). This starts a chain, and the state becomes Closed.
3. Resolution: Since Rebuke is an [Action] spell, no [Action] spells can be played in response to it. Once both players pass priority, Rebuke resolves, and your unit is returned to your hand.
4. State Change: After Rebuke finishes resolving and is cleared from the chain, the chain is empty, and the state returns to Open.
5. Your Window: Because the state is now Open, you have regained the ability to play [Action] spells. You can now play your Hextech Ray to target their unit.

Important Rules Context

• Action vs. Reaction: As you noted, Hextech Ray and Rebuke are both [Action] spells. [Action] spells can only be played when the state is Open (no chain exists). They cannot be played in response to other [Action] spells.
• Showdown Timing: According to the <showdown_flow>, a Showdown does not end until both players pass focus while the state is Open. Because the state returns to Open after Rebuke resolves, the Showdown is still ongoing, and you are free to play your own [Action] spells before the Showdown concludes.
• Focus: After a chain resolves, focus automatically passes to the other player. If you were the one who did not play the last action, you will have the opportunity to play an action once the state opens up again.

This interpretation is consistent with the provided FAQ #1046, which confirms that you can continue to play [Action] spells during a Showdown as long as the state is Open, even if your units have been removed or killed.