Got feedback from the police.
Here's the map:
https://www.google.pl/maps/@52.2231161,21.0156817,60m/data=!3m1!1e3?hl=en
1) Before the junction there are constant lines - no lane changes.
2) under the Polish road code, no lane changes are allowed at the junction - even though no lines are painted where the roads cross
3) immediately after there is a broken line on my side and a constant line on his side; he cannot cross into my lane, but this is exactly where he first indicates and starts pushing into my lane
The reason why the lines don't allow him to change the lane is that the left lane is usually packed by cars turning left (not the case that day), so the natural flow is to drive forward and smoothly merge at the mouth of the next road, where the usual right of way applies (there I would've had to let him through). As the coach driver obviously muscled his way much earlier, exactly where the constant line started, the case will go to court and a penalty will be imposed. In urban areas you cannot overtake on the right anyway - as he might've attempted just after the traffic lights. Mere politeness would've sufficed - just slow down and indicate. I'm not in a hurry.