Oh boy, that canard always comes up.
There was no cooperation there. The Nazis couldn’t care less what Poland did there. And yes, the Czechs actually stole that region in 1920, when Poland was, you know, busy saving the rest of the continent from Bolshevism.
Not saying Poland was right to do it, but this notion that it was in cooperation with the Nazis is disingenuous at best.
We are now treading on relative territory. The Czech very much might have stolen Szczecin from Poland in 1920 but the Soviets might say Poland stolen the Molotov-Ribbentrop areas from Ukraine in 1920 too. That is how it works. We are either respecting established borders or not.
Someone mentioned here that Russia has stolen Crimea and might steal Narva but both Narva and Crimea are historically Russian based on the same logic you apply to Szczecin recognizing it as historically Polish although established borders say otherwise.