I don't really know how to phrase the question, but to put it as clearly as I can, I don't get why it is the P wave "flattens down" when the atria have completely depolarized. I get that the charge is uniform across the atria, but the ventricles are still polarized (and therefore of a different charge compared to the now completely depolarized atria), wouldn't this create a difference in charge and therefore a voltage to produce a current? I think this GIF can help explain my question better:
At frame 12:
I don't understand why there's no current flow and no deflection on the ECG leads despite the charge difference. Rather, why does the GIF deal with the current and charge difference in the atria and ventricles separately? I thought this all might be because of the fibrous cardiac skeleton, which acts as an electrical insulator, but that doesn't make complete sense either; sure it prevent current flow within the heart, but it wouldn't influence current flow outside of it, right?