Comment 4 for bug 1249210

Working in r1196:
session_mediator.cpp: line 212

+++ client_buffer_resource[surf_id].reset();
        std::tie(client_buffer, need_full_ipc) = advance_buffer(surf_id, *surface);

Addition of this reset call fixes it. This was just a guess from the text diff though. I'll come back to understand what is happening after lunch.