`open_video`'s `cursor_blend` argument was a request with no answer: lib.rs did
`let _ = cursor_blend;` and only three backends ever read `CapturedFrame::cursor`.
So a session could ask for a composited pointer, get a backend that silently
discards it, and stream with no mouse cursor and nothing in the logs. Two
separately-confirmed audit findings — the VAAPI dmabuf path and the libav-NVENC CUDA
path — are symptoms of that one hole.
`EncoderCaps::blends_cursor` makes it a fact each backend states. The four exhaustive
`EncoderCaps { .. }` constructors mean adding the field is a compile error until every
backend answers, which is the enforcement mechanism for future backends rather than a
side effect. Vulkan Video answers from its ACTUAL configured source rather than
statically: only the CSC path composites (`prep_cursor` feeds the compute shader),
while the RGB-direct/EFC front-end and the native-NV12 source have no compositing
stage at all and merely warn once that the pointer is being dropped.
`open_video` warns when a session asked for blending and the opened backend cannot
deliver it. A warning is deliberately all it does: `open_video` cannot re-plan
capture, so refusing would trade a missing pointer for a dead session. The host owns
`plan.cursor_blend` and is the only layer that can fall back to capturer-side
compositing — this gives it something to base that on.
Enforcement is NOT included. The reviewed design proposed refusing the client's
host-composite flip to keep the client drawing its own pointer, but `CursorRenderMode`
is client->host only: there is no host->client counterpart, so refusing yields no
pointer at all — the same failure it claimed to prevent.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
enricobuehlerandClaude Opus 5
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