Record two device lessons that cost real time

adb shell input tap is not trustworthy on this phone. It produced six
rounds of false conclusions in one session, including a confident
diagnosis that pointer events were dead which was actually a re-locked
screen eating the input. The CSS-to-device mapping is not css times
devicePixelRatio: 735 CSS px at dpr 3 is 2205 device px inside a 2460px
screen, and the 255px of system bars means the offset must be measured.
Two rival offsets both failed. When a known-good control looks dead,
check the lock state before suspecting the code.

svc power stayon usb writes stay_on_while_plugged_in = 2, and this phone
charges over AC, so it silently never engages. stayon true writes 7 and
works. The 30s screen timeout is a separate setting.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -211,6 +211,28 @@ fingers routinely lift outside the key they went down on.
 bugs. The cost is that a mistyped class renders with no class at all rather than
 failing, which is why the layout is pinned by tests instead.
 
+**Do not trust `adb shell input tap` on this device.** It cost six rounds of
+false conclusions in one session: taps that hit a neighbouring key, taps that hit
+nothing, and an entire diagnosis of "pointer events are dead" that turned out to
+be a re-locked screen swallowing the input. The CSS-to-device mapping is not
+simply `css × devicePixelRatio` — the viewport is 735 CSS px at dpr 3 = 2205
+device px inside a 2460px screen, and the remaining 255px of system bars means
+the offset has to be measured, not assumed. Two rival offsets both failed.
+
+When a *known-good* control appears dead, check the lock state before suspecting
+the code. For anything touch-related, one report from a person holding the phone
+beats an afternoon of synthetic input.
+
+**Stay awake needs `svc power stayon true`, not `usb`.** The `usb` form writes
+`stay_on_while_plugged_in = 2`, and this phone charges over AC (`USB powered:
+false`), so it silently never engages — `mStayOn` stays false. `true` writes 7
+(any source) and works. The screen timeout is separate and was 30s:
+
+```
+adb shell svc power stayon true
+adb shell settings put system screen_off_timeout 1800000   # restore: 30000
+```
+
 **Seeing the UI without the phone.** There is no Chromium on this machine, but
 Firefox can screenshot headlessly with no extra dependency: