doc: screen coordinate reference (screen vs display vs app-local px)

The coordinate-space confusion (screen px 1080x2400 for input tap/
screencap, display px 900x2000 for reading PNGs, app-local px offset by
the 128px status bar, pt at density 2.625) cost real debugging time.
Wrote it down in doc/README.md as the permanent reference:

- the four spaces + conversions (display x1.2 = screen; screen Y-128 =
  app-local; /2.625 = pt)
- the practical tap rule: read (x,y) off the PNG, x1.2, input tap --
  screenshot px ARE screen px, no 128 offset for tap-what-you-see
- fixed geometry (status/nav bars, Gboard rows, editor region in pt),
  with re-measure guidance
- tap-test pitfalls (first-tap-swallowed, first input text can fail,
  wrong-place text = cursor/selection suspect not tap math, don't
  memorize drifting content positions)

All keyboard row values re-measured from a fresh screenshot and verified
live: tapping the Q key at (62, 1712) and backspace at (990, 2020) both
landed first try (file content confirmed). This also exposed that the
old notes had mixed display-px row values with screen-px key values in
the same list -- exactly the confusion this section eliminates.

Also fixed the stale Phase 6 'letterboxed window' note in
development_plan.md (window is full-screen; only the 128px status-bar
offset is real; content positions drift; the OpenFileChan logcat line
was removed) and pointed emu.sh's tap help at the x1.2 rule.

doc/README.md

@@ -74,6 +74,55 @@ Always stop with `down` — SIGKILLing the emulator corrupts the snapshot.
 App package/activity: `pad.pad / org.gioui.GioActivity`. Default root
 directory: `/storage/emulated/0/Notes`.
 
+## Screen coordinates
+
+Three coordinate spaces are in play; mixing them up is the #1 cause of
+tap-test errors. (Measured 2026-08-16 on `pad_avd`; re-derive with
+`adb shell wm size`, `adb shell wm density`, `dumpsys window`, and a fresh
+screenshot with the keyboard open if the device config ever changes.)
+
+| space        | size      | used by                                  |
+|--------------|-----------|------------------------------------------|
+| screen px    | 1080×2400 | `adb input tap`, `screencap` output      |
+| display px   | 900×2000  | what you see when reading a PNG (vision) |
+| app-local px | 1080×2272 | what Gio receives (`evtPos`)             |
+| pt           | —         | app layout units                         |
+
+Conversions:
+
+- display → screen: **× 1.2** (both axes)
+- screen → app-local: **Y − 128** (surface starts below the status bar; X unchanged)
+- app-local px → pt: **÷ 2.625** (density 420)
+
+**The tap rule (covers ~95% of needs):** to tap something visible, read its
+(x, y) off the PNG you are looking at, multiply both by **1.2**, and
+`input tap x y`. Screenshot px *are* screen px — no 128 offset involved.
+The 128 offset only matters when converting screen ↔ app-local (e.g.
+predicting which text line a tap will hit, or reasoning about `evtPos`
+values). The app itself is internally consistent — rendering and input
+share the same origin — so the offset only bites at the adb/screenshot
+boundary.
+
+Fixed geometry (screen px unless noted):
+
+- status bar: top 128 px; nav bar: bottom 63 px
+- Gboard (no suggestion bar): keyboard top ≈ 1518; rows QWERTY ≈ 1716,
+  ASDF ≈ 1880, ZXCV ≈ 2020 (backspace key ≈ (990, 2020)); space/enter
+  row ≈ 2168; icon toolbar ≈ 1548. Re-measure from a screenshot if Gboard
+  changes.
+- App space (pt): editor region starts at (10, 62); line height 16.8 pt.
+
+Tap-test pitfalls:
+
+- The first tap right after launch, file open, or logcat clear is often
+  swallowed — re-tap before concluding anything is broken.
+- The first `input text` after a cursor move can silently fail — retype.
+- `input text` goes to whatever has IME focus; if typed text lands in the
+  wrong place, suspect cursor/selection state (see the GlyphLayout
+  window-relative invariant in `architecture.md`), not tap math.
+- Don't memorize content positions (file rows, buttons) — they drift with
+  content. Measure from the current screenshot with the ×1.2 rule.
+
 ## On-device observation loop (quick reference)
 
 Gio renders into one GL surface, so the authoritative debug signals are
@@ -82,9 +131,10 @@ four):
 
 - `logcat -s pad.pad` — app log (errors, limits, recovery; the normal path
   is quiet by design).
-- `input tap|swipe|text` — drive the UI (see `development_plan.md` for the
+- `input tap|swipe|text` — drive the UI. **All tap coordinates are screen
-  tap coordinates and IME-tap cadence that works). The first tap right
+  px: read the position off the screenshot, ×1.2** (full reference:
-  after launch/open is sometimes swallowed — re-tap.
+  §Screen coordinates). The first tap right after launch/open is sometimes
+  swallowed — re-tap.
 - Autosave debounce is 1 s: **wait ~1.6 s before reading a file back from
   disk** after typing.
 - Memory: `adb shell dumpsys meminfo pad.pad` (watch PSS/RSS; the 3.8 GB

doc/development_plan.md

@@ -281,13 +281,12 @@ scroll (bounded high-water mark from the shaper glyph cache + Go heap; grows
 ~13 MB over the first ~30 scrolls then flat) — **no leak**, well under the 2.5 GB
 OOM line. The profiler's own overhead is negligible (same plateau with it off).
 
-**Test-harness gotcha:** the 390×844 Dp window is **letterboxed** on the
+**Test-harness gotcha:** on-screen tap coordinates are not the naive
-1080×2400 screen (~28 px left / ~133 px top offset), so on-screen tap
+Dp→px map — see `README.md` §Screen coordinates (screen px vs display px
-coordinates are offset from the naive 1:1 Dp→px map. The editor back-arrow hits
+vs app-local; the tap rule is "read off the screenshot, ×1.2"). Don't
-at ~`(86, 264)` px, not the glyph's apparent 1:1 position. Browser file rows
+memorize content positions (file rows etc.); measure them from the current
-(newest-first) start ~y=520 px, ~134 px apart. Verify every open via the exact
+screenshot. Verify file opens from the title bar / file content, not
-logcat line `Logic: OpenFileChan /storage/emulated/0/Notes/<file>`, not a loose
+logcat (the open log line was later removed as noise).
-`OpenFileChan` match.
 
 ### Phase 7 — tap-to-position-cursor verification + fix — DONE (2026-08-16)
 

scripts/emu.sh

@@ -8,9 +8,13 @@
 #   scripts/emu.sh app start|stop|restart
 #   scripts/emu.sh shot [FILE]           # screenshot (default /tmp/pad_shot_<ts>.png)
 #   scripts/emu.sh log [LINES]           # last N lines of app logcat (default 20)
-#   scripts/emu.sh tap X Y               # tap (note: the first tap right after
+#   scripts/emu.sh tap X Y               # tap at SCREEN px (1080x2400 space).
-#                                        # launch/open is sometimes swallowed —
+#                                        # Tap-what-you-see rule: read (x,y) off
-#                                        # re-tap if nothing happens)
+#                                        # the PNG you are viewing and multiply
+#                                        # both by 1.2 (vision displays
+#                                        # screenshots at 900x2000). Note: the
+#                                        # first tap right after launch/open is
+#                                        # sometimes swallowed — re-tap.
 #   scripts/emu.sh type TEXT             # type into the focused field (spaces ok)
 #   scripts/emu.sh cmd <top|bottom|frac F|dp N>   # one-shot editor debug command
 #   scripts/emu.sh perf on|off           # enable/disable the in-app profiler