Jank A/B script for the phone (scripts/jank_ab.sh)

One-shot: install release APK, verify thread priorities (thumb-* nice 2,
slint vsync nice 0), then two 30 s user-scroll windows — real textures vs
grey 1x1 images — with SCRDBG cadence analysis (on-grid %, stalls, p95,
worst). Decisive fork: real-janky+grey-smooth -> texture/GPU path; both
janky -> base render/present path; both smooth -> priority fix worked.
Dry-run tested on the emulator (grey knob detection, priority readback).

scripts/jank_ab.sh

@@ -0,0 +1,85 @@
+#!/usr/bin/env bash
+# One-shot jank A/B for the phone (usage: ADB_SERIAL=10.0.0.5:5555 ./scripts/jank_ab.sh)
+#
+# What it does:
+#   1. Installs the release APK (build with ./scripts/build.sh aarch64 first).
+#   2. Verifies thread priorities (thumb-* at nice 2, slint vsync at 0).
+#   3. Runs TWO 30-second scroll windows — real textures, then grey 1x1
+#      images — with logcat capture + cadence analysis in between.
+#      THE USER MUST SCROLL WITH A FINGER during each window
+#      (moderate drag, no fling — the reported repro).
+#   4. Prints a comparison: % of scroll events on the 8.3 ms grid,
+#      stall count (gap > 16 ms), worst gaps.
+#
+# Interpretation:
+#   real janky + grey smooth  -> texture/GPU path is the culprit
+#   both janky                -> base render/present/event path
+#   both smooth               -> the priority fix worked; ship it
+
+set -u
+E="adb ${ADB_SERIAL:+-s $ADB_SERIAL}"
+APK=target/release/apk/gallery.apk
+PKG=wow.gmp.gallery
+CMD=/sdcard/Download/.gallery_cmd
+GREY=/sdcard/Download/.gallery_grey
+
+[ -f "$APK" ] || { echo "build first: ./scripts/build.sh aarch64"; exit 1; }
+
+scroll_window() { # $1 = label, $2 = tag, $3 = seconds
+  local out="/tmp/jank_$2.txt"
+  $E logcat -c
+  echo "### $1 — scroll now (moderate drag, no fling)..."
+  $E logcat 2>/dev/null | grep -m 2000 "SCRDBG\|TICK \|APLY \|raised\|not found\|panic" > "$out" &
+  local LP=$!
+  sleep "$3"
+  kill "$LP" 2>/dev/null
+  python3 - "$out" << 'EOF'
+import re, sys
+ev = []
+for line in open(sys.argv[1], errors="ignore"):
+    m = re.search(r"SCRDBG Instant \{ tv_sec: (\d+), tv_nsec: (\d+) \}", line)
+    if m: ev.append(int(m.group(1))*1000 + int(m.group(2))/1e6)
+if len(ev) < 10:
+    print(f"  {sys.argv[1]}: only {len(ev)} scroll events — not enough data"); sys.exit()
+gaps = [ev[i+1]-ev[i] for i in range(len(ev)-1)]
+on_grid = sum(1 for g in gaps if 7.5 <= g <= 10.5)
+stalls = [g for g in gaps if g > 16]
+print(f"  events={len(ev)}  on 8.3ms grid={100*on_grid/len(gaps):.0f}%  "
+      f"stalls(>16ms)={len(stalls)}  p95={sorted(gaps)[int(len(gaps)*0.95)]:.1f}ms  "
+      f"worst={max(gaps):.1f}ms")
+EOF
+}
+
+check_prios() {
+  local pid; pid=$($E shell pidof $PKG)
+  echo "### thread priorities (pid $pid)"
+  $E shell "for t in /proc/$pid/task/*; do c=\$(cat \$t/comm 2>/dev/null); case \$c in slint*|thumb*) n=\$(awk -F')' '{print \$2}' \$t/stat | awk '{print \$17}'); echo \"  \$c nice=\$n\";; esac; done"
+}
+
+install_and_start() { # $1 = "grey" to enable the grey knob before launch
+  $E install -r "$APK" >/dev/null
+  if [ "$1" = grey ]; then $E shell "touch $GREY"; else $E shell "rm -f $GREY"; fi
+  $E shell am force-stop $PKG
+  $E shell am start -n $PKG/android.app.NativeActivity >/dev/null
+  sleep 8
+}
+
+echo "=== installing $APK ==="
+install_and_start
+check_prios
+echo
+echo "=== window 1: REAL textures (30 s) ==="
+scroll_window "real" real 30
+echo
+echo "=== restarting with grey 1x1 images ==="
+install_and_start grey
+check_prios
+echo
+echo "=== window 2: GREY images (30 s) ==="
+$E logcat -d 2>/dev/null | grep "grey images ON" || echo "  WARNING: grey knob not active!"
+scroll_window "grey" grey 30
+
+$E shell "rm -f $GREY $CMD"
+echo
+echo "done. Compare the two summaries above."
+echo "  (raw captures: /tmp/jank_real.txt /tmp/jank_grey.txt)"