v5's verdict was `max - min` over the sampled fd counts with a default tolerance
of 0. An encode worker's fd count legitimately moves by one when a dmabuf fd is
in flight at the sampling instant, so the spread was permanently 1 and the leg
failed on a perfectly healthy box — reported, like every red leg here, as "a
shipping blocker, not a flake".
Measured on home-nobara-1 (KDE, RTX 5070 Ti), 33 samples over 480 s:
54 54 54 54 54 54 54 54 55 55 54 54 54 55 54 54 55 54 54 54 55 54 …54
It oscillates and ENDS on 54, exactly where it started. Nothing accumulates.
The replacement is median-of-thirds: median(last third) - median(first third).
That is strictly MORE sensitive to what R2 is actually about — a steady leak
moves the trend just as much as it moves the spread, while bounded jitter moves
only the spread — so this is not the tolerance being widened to get a green.
The spread is still printed, now labelled as jitter when the trend is flat. The
warm-up window already covers the one-off first-sight-of-each-buffer cost, so a
plateau inside it is by design not a leak; a step that never comes back still
trends and still fails.
The self-test grows the cases that force this to be a real assertion: the
measured oscillation must trend to zero, a synthetic leak must still trend up, a
flat series must be flat, and a step that never returns must be caught. Writing
them is what caught my own arithmetic — the first draft asserted a leak trend of
12 where the reader correctly says 10.
Also records what the v5 log now makes obvious: `--minutes` does NOT set the wall
clock. `spike` is frame-count bounded (`seconds * fps`), and a KWin virtual
output being driven hard delivers ~197 fps against a `--fps 60` budget, so a
"10 minute" run ended after 182 s. Ask for more minutes than you want.
enricobuehler
890b67a863