I could have won at my upkeep after being milled out during my opponent's turn. The game prevented me from taking instant-speed actions at my upkeep.

My opponent's board state was as follows:
1x Riverchurn Monument
9x Lands (6 tapped; 1x Swamp and 2x Bleachbone Verge untapped)

My board state was as follows:
1x Biotech Specialist
1x Golem creature token
2x Weapons Manufacturing
5x Munitions artifact token
2x Lander artifact token
1x Nutrient Block
1x Legion Extruder
6x Lands

In hand I had Torch the Tower and Legion Extruder. My opponent had 5 cards in hand.

My opponent milled me out during their turn by playing and exhausting a Riverchurn Monument. I tapped out to sacrifice a munitions token to Legion Extruder, dealing 2 damage to them directly from the token and another 2 damage from Biotech Specialist, bringing them to 5 life. At this point, I saw the cog in the upper righthand corner of the screen disappear, signaling the game has deemed the game is over and I could not do any instant-speed actions.

However, before my draw step where I would lose the game, I could have sacrificed both Lander tokens and sacrificed a Munitions token to Legion Extruder to do a total of 8 damage to my opponent, allowing me to (presumably) win the game, barring any instant speed life gain my opponent could have had for three mana.

I should not have to press control for this, as it should act similarly to the "Draw" prompt like Sheoldred has. If I'm going to lose at the draw step, but I can still do actions, I should be able to do those actions. Perhaps it could be a special timer since a loss is inevitable if nothing can be done, but it should not skip directly over upkeep when instant-speed actions can change the course of the game.

I was unable to do any of these actions that would have won me the game.