An injected touch does not wake a blanked screen, and every gesture arm then measures a still image

scope: generic · severity: trap · confidence: proven · subsystem: display

Evidence

taimen 2026-09-05 – with the panel blanked, ph-gesture-bench.py drag 720 2400 720 900 500 3 moved window.scrollY 0 -> 0 and counted 0 frames in 10.2 s; the identical drag after org.gnome.ScreenSaver.SetActive false moved it 0 -> 1721. Writing bl_power=0 as root lit the backlight and the same drag still moved 0 px

Symptom — a gesture arm that ran, printed no error, and measured nothing:

drag         on org.gnome.Epiphany
  NOT MEASURED: 0 frames in 10.2s. The screen never changed during the
  run -- the gesture hit nothing that animates.
[rec] scrollY 0 -> 0

Or, worse, an arm whose instrument does not check: uprobes armed on the render phases report zero hits for every phase, which reads exactly like “this workload does not use the rendering pipeline”. A whole session’s conclusion was built on that null, and it was wrong – see the-scroll-stall-is-the-pages-own-javascript.

Do not assume a phone that answers ssh has a screen that is on. phosh’s screensaver powers the output down after a few minutes idle, and uinput events do not undo it: the injected touch reaches the client, but never the idle notifier, so the compositor keeps the output off. An unattended arm – which is every arm, since it takes minutes and nobody is watching the panel – starts blanked by construction.

Nothing in the numbers says so. The DPU counter reads 0 frames, which also happens for a gesture aimed at an end stop; grim blocks and times out, which also happens on a wedged compositor; and the phone is otherwise perfectly healthy.

The backlight is not the lever. echo 0 > /sys/class/backlight/*/bl_power as root turns the backlight on and changes nothing else: the compositor’s output stays off and the same drag still moves the page zero pixels. Measured, not assumed.

The fix is the screensaver’s own call, and it has to run as the session user – from root the session bus answers Call failed: Socket not connected:

busctl --user call org.gnome.ScreenSaver /org/gnome/ScreenSaver \
    org.gnome.ScreenSaver SetActive b false

tools/ph-ui.py unblank is that call, dropping back to the session user with sudo -u when it is invoked as root, and ph-gesture-bench.py now runs it before every measurement – so every arm in this tree that drives a gesture is covered without changing the arm. If you are driving the screen some other way, call it yourself before you believe a frame count.

Related — the-instrument-is-guilty-until-proven-innocent, uprobes-do-not-attach-to-an-already-mapped-library.