A sender MUST NOT allow a flow-control window to exceed 2^31-1
octets. If a sender receives a WINDOW_UPDATE that causes a flow-
control window to exceed this maximum, it MUST terminate either the
stream or the connection, as appropriate. For streams, the sender
sends a RST_STREAM with an error code of FLOW_CONTROL_ERROR; for the
connection, a GOAWAY frame with an error code of FLOW_CONTROL_ERROR
Currently, TS accepts such a invalid flow control.
It would cause overflow but is not critical, so I'm setting issue type as Improvement.