feat(dgw): keep sparse live shadow streams moving

[irvingouj@Devolutions (irvingoujAtDevolution)]

RDP sources only send a frame when the screen repaints, so a live viewer can go seconds or minutes without a frame. Browsers then show nothing until the next frame and can freeze far ahead of the data. While a live clip is quiet, Gateway can now repeat the last picture at a fixed interval, trailing the source a little so a late frame isn't overtaken. Only live viewers are affected; the stored recording is unchanged.

[github-actions]

Let maintainers know that an action is required on their side

• Add the label release-required Please cut a new release (Devolutions Gateway, Devolutions Agent, Jetsocat, PowerShell module) when you request a maintainer to cut a new release (Devolutions Gateway, Devolutions Agent, Jetsocat, PowerShell module)

• Add the label release-blocker Follow-up is required before cutting a new release if a follow-up is required before cutting a new release

• Add the label publish-required Please publish libraries (`Devolutions.Gateway.Utils`, OpenAPI clients, etc) when you request a maintainer to publish libraries (Devolutions.Gateway.Utils, OpenAPI clients, etc.)

• Add the label publish-blocker Follow-up is required before publishing libraries if a follow-up is required before publishing libraries

[irvingouj@Devolutions (irvingoujAtDevolution)]

Draft, experiment state. Report with all numbers: kept locally, summary here.

• Off by default. DGATEWAY_SHADOW_FILL_MS / DGATEWAY_SHADOW_FILL_DELAY_MS turn it on (env vars are experiment-only, a real config option comes later). 500/500 is what I'd ship.
• Chromium shadow-player on lab MsRdpEx recordings, loopback Gateway: off → first picture 29 s, 51-86 % of samples stalled; 500 ms → first picture 2.7 s, flat 2.6 s lag, no stalls; +500 ms delay → 3.1-3.4 s flat. Cost +50..85 kbps per viewer.
• With injected push jitter, the 500 ms delay keeps late source frames from being overtaken up to 300 ms of jitter (0 vs 8-12 moved), 3-5 left at 1 s.
• Also recovers frames the BestPerformance encoder drops (75 in → 54 out over 180 s); without it a dropped change can stay invisible until the next repaint.
• examples/sparse_fill.rs is the timed replay harness, and the late_frames_moved counter is only there for it. Both go before this leaves draft.
• Depends on fix(dgw): start shadow viewers at the live picture #2061 for the E2E rows to mean anything (join backlog dominated them).

Note

Human-tuned, LLM-assisted content.