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Live medium-quality stream errored with "negotiation problem" on the host and rendered a squashed, garbled picture in the viewer. Two causes, both from inserting videoscale before videoconvert with PAR+range caps: - videoscale was scaling pipewiresrc's raw output directly. The portal source's format/memory (e.g. DMABuf) isn't something software videoscale negotiates — the original pipeline always fed pipewiresrc through videoconvert first. Move videoscale *after* videoconvert so it operates on system-memory NV12/I420. - `pixel-aspect-ratio=1/1` + a width range over-constrained negotiation and risked a non-square-PAR / distorted result. Instead compute an exact even WxH from the known source dimensions (Wayland: portal size; X11: root/window geometry), preserving aspect, and pin it fully in the caps. This is also downscale-only now — a source already at/below the target height is left native instead of upscaled. Unknown dims (rare X11 geometry failure) fall back to the height-only + square-pixel + even width-range negotiation. source_dims threaded through pipeline::spawn from both backends. Smoke test updated to mirror the new ordering (1920x1080 -> 852x480, videoscale after videoconvert) and still asserts an even sub-source width. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>