Is it mandatory in the presented situation to stop before entering the intersection?

In the video you are driving along a street in town behind a navy-blue car towards a junction with traffic lights; above your left-turn lane, directional traffic lights S-3 show a green arrow, and above the other lanes traffic lights S-1 show a green light; before the junction there is a pedestrian crossing, and a green-and-yellow bus driving in front of the navy-blue car is turning left at the junction. You do not have to make a mandatory stop before entering the junction - the green light allows you to enter. The answer is: no.

The Regulation on road signs and signals states that a green signal on traffic lights S-1 means permission to drive beyond the traffic lights, and light signals take precedence over signs that regulate right of way; an obligation to stop arises only from a red or amber signal or from sign B-20. However, the Road Traffic Act prohibits entering a junction if there is no room beyond it to continue driving, and requires particular caution at a junction and a crossing. So green does not mean driving blindly: you watch the bus turning left, whose rear may still stick out into the junction, and the navy-blue car braking before the crossing, check the crossing and enter without stopping only when the road beyond the junction is clear.

Legal basis: § 95 para. 1 point 1 and § 97 para. 1 of the Regulation on road signs and signals; Art. 5 para. 3, Art. 25 para. 1 and 4 point 1 and Art. 26 para. 1 of the Road Traffic Act (Prawo o ruchu drogowym)