True while bytes accepted by send exceed the transport's
high-water mark and have not yet been handed to the OS — the peer
(or the pipe to it) has stalled. Hosts defer frame PRODUCTION while
this is true. A live session never selectively drops accepted frames
(the wire protocol's diffs are only valid against the exact previous
frame the peer holds); exceeding a transport hard bound fails the whole
session closed.
Send bytes accepted but not yet handed to the OS before the
transport reports itself backlogged. 256 KiB holds a few frames of
a busy dashboard without tripping, while bounding app-side memory
when the peer stalls (the kernel socket buffer sits below this).