Teach the X11 fixture about the MATE atom and verify import
preference, source target advertisement, and copied-files delivery.
Cover the same Wayland offer and source behavior. Assert that MATE
reuses the single GNOME-style payload instead of fetching a duplicate
representation.
Exercise the core parser and generator with the MATE copied-files MIME,
including invalid operation handling and byte-identical remote output.
Mark the last non-empty FILE_DATA payload with BEGIN and END when the
producer knows it is terminal. Complete producer bookkeeping in the
following fileDataEnd call without consuming another LGMP grant. Keep
the standalone terminal record for empty and legacy unknown streams.
Drain up to 64 queued clipboard records per worker pass instead of
sleeping after every message. Poll at 1 ms while transfers, blocked
writes, held input, or pending output are active, and retain the 10 ms
interval while idle.
This removes a serialized grant round trip from every non-empty file
response and reduces scheduling delay while a large transfer is active.
Keep the 1 MiB payload size because the bottleneck was stop-and-wait
latency, not the chunk capacity.
Exercise a full 1 MiB response in a single BEGIN|END grant and retain
coverage for empty unknown-size responses.
