gallery/HANDOFF.md

# Gallery — development hand-off

A high-performance image gallery for Android (primary), Linux and macOS,
written in Rust with the **Slint** UI toolkit. This document is the starting
point for development: it captures every decision, gotcha, and measurement
from the framework evaluation that preceded it, so no learning has to be
rediscovered.

**Status: M3 done (2026-10-27, twice overhauled)** — M0 scaffold, M1 metadata
scan + sqlite + instant list paint, M2 thumbnail engine (turbojpeg DCT-shrink
decode, worker pool, disk cache, cancellation), M3 grid UI (3-column ListView
grid, shimmer placeholders, memory-bound cell retention) all verified: 15
unit tests (incl. two queue-priority regressions), 5k synthetic engine test,
and on a real Pixel 9 Pro (8,577 photos: 60 fps, 0 hitches). The thumbnail
loader was rewritten TWICE on 2026-10-27 because "stop → long shimmer → small
move → everything pops" kept surviving: first the scroll handler forgot cells
when scrolling stopped (fix: **viewport reconciliation** — the 10 Hz tick is
the sole authority, the invariant "every viewport cell is painted / in flight
/ failed" is restored unconditionally each tick and `debug_assert`ed), then
the worker-level FIFO still let stale transit work delay a fresh viewport
(fix: **worker-queue prepend + cap** — bursts are prepended ahead of older
queued work; per-worker cap drops the oldest tail with a `skipped` result).
Stress-verified on the emulator (debug build) + phone before shipping:
fast up/down 0–120 ms to full coverage, cold landings bounded by decode
throughput, chaos scrolling max oldest in-flight job 217 ms (was 8 s),
0 invariant violations in ~1,450 ticks, 0 panics/ANRs. Next: M4 viewer.

---

## 1. Goals

- Browse and view a local photo collection (thousands of images) that stays
  smooth: instant grid, no decode jank, cached thumbnails.
- Full-screen viewer with pinch-zoom/pan.
- Correct EXIF orientation, HEIC support (Android camera default).
- Same codebase on macOS + Linux (desktop `cargo run`) and Android
  (`cargo apk2`).
- Quality of the *platform experience* matters: safe-area insets, IME-free
  UI, native-feeling widgets, animations — first-class support preferred over
  manual patching.

Non-goals (for now): editing, cloud sync, video, widgets/live-updates.

## 2. Why Slint (decision summary)

Full evaluation in `/mnt/Backup/prog/android/ui/REPORT.md` (same tree:
`~/prog/android/ui/REPORT.md`), raw notes in `NOTES.md` there. Headline:

| | Slint 1.18.1 | eframe/egui 0.37.0 |
|---|---|---|
| Virtualized list (the gallery's lifeline) | ✅ `ListView` std-widget, only visible rows instantiated | ❌ `ScrollArea::show` lays out all children; `show_rows` = manual virtualization API |
| Async image → UI | swap `slint::Image` into a property; only that tile redraws (retained mode) | `ctx.load_texture` + custom loader; visible set rebuilt every frame |
| Lifecycle hook for permissions | ✅ `init_with_event_listener` (Start/Resume/Pause/Stop/SaveState/...) | ❌ none in `App` trait (winit swallows it) |
| Android safe-area | ✅ `safe-area-insets` built in | manual 32px-ish panel workaround (winit#3910) |
| Animation | ✅ declarative built-in engine | hand-rolled |
| IME text entry | ✅ | ✅ |
| APK size (release aarch64) | 8.57 MB | 2.87 MB |
| Clean build time (x86_64, this machine) | 195 s / 270 crates | 76 s / 137 crates |
| Desktop Linux system deps | `libfontconfig1` + `pkg-config` | none |
| License | royalty-free for proprietary mobile/desktop (embedded excluded) / GPLv3 / commercial | MIT OR Apache-2.0 |
| Min Android API | 26 | 24 |

For a stateful, form-and-grid app, the declarative model was the right fit
(see the discussion about `pad`'s immediate-mode frame friction); egui wins
for games/dashboards and on APK size. Known Slint caveat: an open upstream
"abort on pointer scroll" bug in the Android backend (`todo!()` in the
pointer-event path) — verify it's fixed in 1.19 before relying on heavy
scrolling; if it fires, the panic is obvious in logcat.

## 3. Environment, build & test loop

Everything below is verified working on this machine (2026-10-27).

```bash
export ANDROID_HOME=$HOME/android-sdk
export ANDROID_NDK_ROOT=$HOME/android-sdk/ndk/27.0.12077973
export PATH="$HOME/.cargo/bin:$ANDROID_HOME/platform-tools:$PATH"

# Rust 1.99.1 via rustup; tools: cargo-apk2 1.5.0, cargo-ndk 4.1.2
./scripts/build.sh debug      # debug x86_64 APK + adb install + launch
./scripts/build.sh            # release x86_64 (emulator)
./scripts/build.sh aarch64    # release aarch64 (device)
cargo run                     # desktop (Linux needs libfontconfig1+pkg-config)
```

- Emulator: `emulator -avd pad_avd -no-window -no-audio -gpu off -no-snapshot`
  (google_apis x86_64, Android 15). **`-gpu off -no-snapshot` matters** —
  swiftshader_indirect + snapshot crashed the emulator once during the eval.
  If it won't start, the host key lost its wireless pairing — see the pairing
  procedure in `~/.pi/agent/AGENTS.md` (phone) / emulator docs (this AVD).
- APK lands at `target/<debug|release>/apk/gallery.apk`.
- Release signing uses the debug keystore
  (`CARGO_APK_RELEASE_KEYSTORE=$HOME/.android/debug.keystore`,
  `CARGO_APK_RELEASE_KEYSTORE_PASSWORD=android`) — fine for personal use;
  for Play, generate a real keystore and point the env vars at it.
- `cargo apk2` quirk: `--target` (not `-t`). `--lib` builds the cdylib.
- Grant "All files access" to the app on the emulator after install:
  `adb shell am start -a android.settings.MANAGE_ALL_FILES_ACCESS_PERMISSION`
  (or Settings UI). `adb install -r` keeps the grant.
- **Emulator test data (M3)**: the AVD's `/Pictures/gallery-test/` has ~5k
  *fake* JPEG stubs (zlib garbage — they scan fine but fail to decode, good
  for volume tests). Real, decodable test JPEGs live in
  `/Pictures/realtest/` (150, generated with PIL). They were `touch`-ed to a
  future mtime so they sort to the TOP of the grid — when reasoning about
  "which cells should have images", remember only cells 0–149 are real;
  everything below is stubs and correctly shows shimmer. **`/Pictures/realtest_extra/`
  holds 700 more real, decodable JPEGs (PIL, 1200–4032 px, mtime 2031 so they
  sort to the very top, above the 150 in `realtest`) — added for the
  scroll-back backlog repro; the real region is now ~850 files (283 rows).
  `adb push` of a directory nests a subfolder (push files individually or
  `mv` up); `adb shell touch -d '…'` works for setting mtimes. The thumb cache is at
  `/sdcard/Android/data/wow.gmp.gallery/files/thumbs` (wipe it to force
  re-decode).
- Perf measurement: `adb shell perfetto` / `adb shell dumpsys gfxinfo wow.gmp.gallery`,
  `systrace`-style capture via Perfetto trace shell. Cold start:
  `adb shell am start -W`.

## 4. Architecture

The UI framework is only the display layer. The performance lives in the
thumbnail engine, and it is framework-agnostic:

```
┌─ metadata ─────────────────────────────────────────────────────────┐
│ sqlite (rusqlite, bundled): path, mtime, size, w, h, orientation,  │
│ thumb_path, date_taken            — one background scan pass       │
└──────────────┬─────────────────────────────────────────────────────┘
               │ model → Slint (list of visible-range items)
┌─ thumbnail pool (tokio-free: N std::threads, N ≈ cores/2) ─────────┐
│ per job: token cancellation (AtomicU64 gen counter) + priority     │
│  1. disk cache hit?  .thumbs/{sha1(path|mtime|size)}.jpg  → done   │
│  2. decode AT TARGET SIZE (never full-size for the grid):          │
│       JPEG: zune-jpeg "turbo" (decode+downscale in ONE pass,       │
│             pure Rust, actively maintained) or turbojpeg (SIMD)    │
│       HEIC: imagepipe (0.5, 2026-08) — VERIFY Android NDK build    │
│       PNG:  image/png                                              │
│  3. apply EXIF orientation (once, here — never per frame)          │
│  4. re-encode small JPEG → disk cache                              │
│  5. channel → UI: (id, slint::Image)                               │
│ prefetch: decode visible ± 1 row; cancel items scrolled out        │
└────────────────────────────────────────────────────────────────────┘
┌─ UI (Slint) ───────────────────────────────────────────────────────┐
│ grid screen:   ListView (vertical) → row of N square Image cells   │
│ viewer screen: full-size async decode (once, cached) + manual      │
│                two-finger pinch transform (no built-in in 1.18)    │
└────────────────────────────────────────────────────────────────────┘
```

Module layout:

```
src/
  lib.rs        # android_main + platform init + scan/UI glue (M1/M2)
  main.rs       # desktop entry                                     [exists]
  main.slint    # screens (split into ui/ components at M3)        [exists]
  db.rs         # sqlite schema + incremental scan apply           [done M1]
  scan.rs       # directory walk, image detection, walk_stream      [done M1]
  thumb.rs      # pool, dispatcher, disk cache, generation cancel  [done M2]
  images.rs     # turbojpeg shrink-decode, EXIF orientation, PNG   [done M2]
  viewer.rs     # full-size load + transform state                  (M4)
  perm.rs       # storage permission check/request (M5)
```

Crate choices (all verified current on crates.io, 2026-10):

| crate | version | why |
|---|---|---|
| `slint` / `slint-build` | 1.18.1 | pinned; upgrade to 1.19 after the pointer-scroll fix lands |
| `rusqlite` | 0.41, `bundled` feature | SQLite without system deps; cross-compiles to Android |
| `turbojpeg` | 1.5.1, `cmake` feature | libjpeg-turbo DCT-level shrink decode (1/2,1/4,1/8) — 48 MP at 1/8 peaks ~2.3 MB vs 137 MB full decode. Vendors + cross-builds libjpeg-turbo via CMake/NDK (verified). Chosen over `libvips` (same DCT shrink mechanism, +6–10 MB of .so files, LGPL, 4.3k downloads) and over `zune-jpeg` (no DCT shrink = full-size decode for 12 MP photos). |
| `image` | 0.25, `default-features=false, png` | PNG thumbs + `imageops::resize` final fit |
| `kamadak-exif` | 0.6 | EXIF orientation tag (all 8 values applied as pixel maps); `experimental::Writer` used only in tests |
| `sha1` | 0.11 | cache keys |
| `imagepipe` | — (deferred) | HEIC: the real device has **0 HEIC** files (9109 JPEG, 27 PNG), so HEIC support is deferred; `image` can gain `heic` or libvips can replace the JPEG path later — the engine API is decoder-agnostic |

Decode sizes: grid tiles render at ~cell×2 (Retina) — e.g. 500 px side.
Full-screen: full decode once (12 MP ≈ 30–80 ms off-thread, fine), cached in
the in-memory LRU until memory pressure.

**The five failure modes the current (unknown) app likely has** — this design
eliminates all five: (1) full-size decode for grid cells, (2) no disk
thumbnail cache, (3) no list virtualization, (4) decode on UI thread without
cancellation, (5) EXIF rotation applied per frame.

## 5. UI plan (Slint patterns)

- **Grid**: `ListView` is the only virtualized std-widget (confirmed in
  source docs: "Only visible rows are instantiated"). Grid = vertical
  `ListView`, each row a horizontal layout of square `Image` cells
  (columns ≈ `floor(window-width / cell)`; recompute on resize).
  Cells bind `image <=> model.image` — when the pool delivers a thumbnail,
  only that cell invalidates (retained mode; no repaint calls to remember).
- **Placeholder→image**: cell shows a `Rectangle` (shimmer optional, M6)
  until its `image` is set.
- **Image cache**: Slint's built-in path-based LRU is 5 MB (`CLruCache` in
  `internal/core/graphics/image/cache.rs`) — too small and the wrong layer
  for us; our cache lives in Rust (disk + compressed-bytes LRU).
  No mipmaps in the Skia renderer — irrelevant since we pre-downsample.
- **Viewer**: no pinch helper in Slint 1.18 (`TouchArea` has no pinch
  callback; grepped the stdlib). Manual two-finger math on `TouchArea`
  (distance/centroid → scale/translation, ~50 lines; crib
  `slint-src/examples/native-gestures` — source checkout is at
  `~/prog/android/ui/slint-src`, tag v1.18.1). Double-tap-to-zoom is a nice
  extra (M6).
- **Screens**: single `Window` with a stacked layout (grid + viewer overlay)
  or a `Navigator`-style state property; keep it simple until M4.
- **Safe area**: always pad with `root.safe-area-insets.{top,bottom}` (done
  in the scaffold).
- **Animation**: use built-in `animate` statements (syntax: `animate <prop>
  { duration: ...; easing: ...; }` — it is a *statement*, not an element).
  Slide/fade for the viewer is a one-liner.

## 6. Slint 1.18 API gotchas (all hit firsthand in the eval)

1. `animate` is a statement (`animate width { duration: 300ms; }`), not an
   `Animation {}` element.
2. Text input widget is `LineEdit`, not `TextField`.
3. Markdown interpolation: `text: "Count: \{count}"` (backslash-brace).
4. `length` is the unit type for lengths (`property <length>`, not
   `logical<length>` in the 1.18 Rust API).
5. No Rust macros/closures inside `.slint` callbacks — callbacks call global
   functions or invoke Rust via `@global` / imported callbacks.
6. 1.18 ↔ 1.19 **docs drift** — when reading docs online, check which
   version; the local source at `~/prog/android/ui/slint-src` (tag v1.18.1)
   is ground truth for the pinned version. `git grep` beats the docs.
7. `slint::android::init_with_event_listener(app, |event: &PollEvent| ...)`
   re-exports the whole `android-activity` crate as
   `slint::android::android_activity`; `AndroidApp` is also re-exported at
   `slint::android::AndroidApp`.
8. `slint::Image` runtime construction: `Image::from_rgba8_premultiplied`
   (also `from_array`); assign to an `image` property to update a cell.
9. Desktop Linux: needs `libfontconfig1` + `pkg-config` (pure Rust on macOS).
10. Retained-mode redraw: changing a property re-renders only affected
    elements — design bindings so thumbnail delivery touches one property.
11. **`in property` is NOT auto-bound to a same-named `for` variable.** In
    `for cell[c] in row.cells: CellView { ... }` you must write
    `CellView { cell: cell; ... }` — otherwise the component's `in property
    <Cell> cell` stays at its default (empty) and the cell renders blank
    while *looking* correct. (This silently broke M3 for an hour.)
12. **Updating a nested model in place does NOT reach a ListView row that
    was recycled while off-screen.** In-place `inner.set_row_data(col, …)`
    updates only the currently-live `CellView`s; a row whose `GridRow`
    instance was recycled keeps showing stale cells until the next scroll.
    The reliable path is to replace the WHOLE outer row:
    `outer.set_row_data(row, Item { cells: ModelRc::new(VecModel::from(…)) })`
    — the outer `row_changed` makes the ListView re-bind `row-data` and the
    fresh nested read picks up the new state. Batch cell changes per row and
    do ≤ ~16 row-replacements per 100 ms tick (see `apply_row_batch`), or a
    fast scroll mass-updates and ANRs.
13. `ListView` has no `spacing`/`gap` property in 1.18 — bake inter-row
    rhythm into the item height (we add a 4 px bottom margin inside
    `GridRow`; `ROW_SPACING` in lib.rs must match).

## 7. Permissions & platform integration (Android)

Declared in `Cargo.toml` (verified schema in cargo-apk2 1.5 / ndk-build2:
`uses_permission` is **top-level** under `[package.metadata.android]`, and
`Permission` supports only `name` + `max_sdk_version`):
`READ_MEDIA_IMAGES`, `READ_EXTERNAL_STORAGE` (maxSdk 32),
`MANAGE_EXTERNAL_STORAGE`.

Runtime flow (M5), using the `ANDROID_APP` clone stashed in `src/lib.rs`:

1. **MANAGE_EXTERNAL_STORAGE** (primary path, same as passgo): no runtime
   dialog exists — check `Environment.isExternalStorageManager()` via JNI,
   if false launch `Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION` via
   `ANDROID_APP.get().run_on_java_main_thread(...)` (attach `JNIEnv` from
   `vm_as_ptr()`; Activity from `activity_as_ptr()`), then **re-check on
   `MainEvent::Resume`** — the hook is already wired in `src/lib.rs`
   (TODO(M5) marker).
2. **READ_MEDIA_IMAGES / READ_EXTERNAL_STORAGE**: `requestPermissions` via
   the same JNI route; the bare `NativeActivity` does not forward
   `onRequestPermissionsResult` to native, so either (a) poll
   `checkSelfPermission` after requesting (good enough), or (b) ship a custom
   `NativeActivity` subclass via cargo-apk2 `java_sources = "android/java"`
   (manifest `activity name` points at it) — the proper callback, reusable
   pattern from the eval.
3. Facts that constrain design (verified in android-activity 0.6.1 source):
   no `NewIntent` event; `poll_events` may only run on the `android_main`
   thread (Slint owns it — don't add a second consumer); `AndroidApp` is
   `Clone` and a clone works from any thread (`run_on_java_main_thread`,
   `vm_as_ptr`, `activity_as_ptr`).
4. **Source discovery** for photos: start with a plain directory scan of
   `/storage/emulated/0/Pictures` + `DCIM` (works with all-files access,
   emulator-friendly, and it's what makes the app testable without
   MediaStore). MediaStore integration (API-aware, scoped to
   `READ_MEDIA_IMAGES`) is a later refinement if directory scanning proves
   insufficient.

## 8. Performance targets & measurement

Targets (emulator is NOT representative for absolute decode times — validate
on a real device with the aarch64 build):

- Grid: 60 fps scrolling at 5k+ items; zero full-size decodes on the hot path.
- First frame: grid of placeholders < 100 ms after launch (metadata from
  sqlite, images async).
- Thumbnail decode (grid size): < 5 ms off-thread per JPEG (measure with
  `std::time::Instant` logs; zune-jpeg turbo should be well under).
- Steady-state memory: in-memory thumb cache bounded (~64–128 MB compressed
  bytes on a phone) + Slint's texture memory for visible tiles only.
- Cold start (emulator, release): compare against the eval baseline
  (Slint sample ≈ 1–2 s; gallery must not regress > 0.5 s).

**Scroll-FPS measurement (real device, verified 2026-10-27):**
`dumpsys gfxinfo wow.gmp.gallery` reports **0 frames** for Slint — the
NativeActivity window renders via its own EGL surface and is not tracked by
gfxinfo on Android 15 (works for normal View apps). Use SurfaceFlinger's
per-layer latency instead:

```bash
# find the layer name:
adb shell dumpsys SurfaceFlinger | grep "VRI-wow.gmp.gallery"
# while scrolling (swipes via `input swipe`): capture 1 s of frame history
adb shell "dumpsys SurfaceFlinger --latency 'VRI-wow.gmp.gallery/android.app.NativeActivity#<id>'"
# each line: latch_time display_time … — display_time is the vsync grid.
# fps = 1e6 / avg(display gap); jank = display gaps > ~25 ms; the latch
# column shows frame PRODUCTION timing (p95 ~17 ms = on time at 60 Hz).
```

Result (Pixel 9 Pro, release aarch64, 8,577 photos, 500 px thumbs):
sustained **60.0 fps, 0 display hitches** while scrolling, both cold cache
(production p95 21–25 ms) and warm cache (production avg 16.6 ms). The
display presents at a 60 Hz vsync grid (Slint requests Default framerate;
120 Hz is a later polish item, not an M3 acceptance criterion).
Note: the shimmer `animation-tick()` keeps frames flowing even at rest,
so latency history is continuous — filter by swiping if you need
scroll-only frames.

Test data (M2): generate a synthetic set on the device — a small Rust `bin`
or Python script that writes N=5,000 JPEGs of varied sizes/orientations into
`/storage/emulated/0/Pictures/gallery-test/` (include a few HEICs if
imagepipe works; include rotated EXIF). Reuse it for every perf run.

Measure with: `adb shell am start -W` (cold start),
`dumpsys gfxinfo wow.gmp.gallery framestats`, Perfetto trace (GPU + main +
pool threads), and `logcat` pool timing logs behind a `debug_timing` flag.

## 8.5 Jank hunt (IN PROGRESS — phone is the acceptance target)

Symptom: user reports janky grid scroll (moderate drag, no fling) on the
phone (Pixel 9 Pro, 120 Hz). Emulator repro via programmatic scroll hook.

### What is ruled out (all measured)
- **App tick cost**: 7–34 µs/tick, even inside 28–39 ms gaps. Not the tick.
- **Logd backpressure**: fixed — `nblog` (bounded mpsc + drain thread,
  nanosecond `try_send` from UI). Idle CPU 71% → 5%.
- **60/120 Hz display zone flap**: SystemUI (uid 10156) was asserting a 60 Hz
  override every minute; pinned with `settings put system min_refresh_rate
  120` (reversible: `settings delete system min_refresh_rate`).
- **Shimmer animation**: `animation-tick()` in a per-cell binding evaluated
  O(ALL 8577 cells) `sin()` every frame (70.9% CPU idle). Now a static
  placeholder; proper O(visible) shimmer is M6 work.
- **Whole-row model replacement**: now in-place `cells.set_row_data` (one
  property change; texture upload only when pixels change).
- **Texture upload CPU**: APLY 1.2–2.9 ms/cell (480 KB) — under budget, and
  too cheap to explain 28–39 ms (would need 12–17 MB/s on a Mali).

### Measured facts
- Emulator, programmatic scroll (`/sdcard/Download/.gallery_cmd` →
  `scroll <px/ms> <ms>`), real textures: 28–146 ms cadence. During every
  stall the UI thread is **100 % on-CPU** (jsample: state R, wchan 0) —
  Slint's render pass is CPU-bound.
- Emulator, 300 items instead of 5852: **equally janky** → scene/item count
  is NOT the cost driver.
- Emulator, grey 1×1 images (`.gallery_grey` touch file): steady ~16.6 ms
  (60 Hz pacing), worst 44 ms → the cost is **rasterizing ~30 live textured
  cells at 1080×2400 through the emulator's software GL (llvmpipe)**.
  The emulator has a hard rendering ceiling and can never scroll this
  smoothly — it validates LOGIC (invariants, paint coverage), not feel.
- Phone: 64 % of frames on the 8.3 ms (120 Hz) grid; ~4 % take 28–39 ms of
  non-tick work with d≈0 (UI thread not in our code). Frequency ≈ every
  170 ms — much faster than new cells enter the viewport (~1.7 s), so it is
  NOT first-paint/upload timing. Suspects left: GPU/present contention
  (Mali + SystemUI both at 120 Hz), Slint EGL/swap path, or an
  event-loop/vsync interaction. Needs the phone.

### Tools / hooks in the tree (all tagged "REMOVE after the hunt")
- `src/lib.rs`: `DbgScroll` + `.gallery_cmd` file (also `stop`);
  knobs `.gallery_max` (item cap) and `.gallery_grey` (1×1 grey images);
  `SCRDBG`/`JANKTEST`/`TICK`/`APLY` logs at `info`.
- `jsample.c` (in /tmp, NDK-cross with
  `~/android-sdk/ndk/27.0.12077973/.../x86_64-linux-android26 -static`):
  per-thread `/proc stat` + state + wchan sampler at 1 ms; usage
  `jsample <interval_ms> <pid> <tid...>`.

### Next (phone, morning)
1. **Decisive A/B on the phone**: `touch /sdcard/Download/.gallery_grey`,
   restart app, user scrolls 30 s, capture logcat. Then remove the file,
   restart, repeat. Grey smooth + textured janky → texture/GPU path is the
   culprit (then: smaller decode target, upload strategy, possibly Slint
   image pipeline). Grey also janky → base render/present path (then: Slint
   window/EGL config, refresh-rate interaction, vsync driver).
2. Run `jsample` over one of the user's real scrolls and wchan-correlate
   the 28–39 ms windows (is android_main R, S/futex, or S/do_epoll_wait?).
3. Compare against Fossify (installed on the phone, user says smooth):
   `dumpsys SurfaceFlinger --latency` on both apps' layers during identical
   scrolls.
4. Candidate code changes (after the A/B): GRID_TARGET 500→400 (one-time
   cache invalidation — don't do it before knowing the culprit), 1 painted
   cell/tick instead of 2, worker nice level (+2) so turbojpeg bursts
   can't preempt the UI thread.

## 9. Milestones

- **M0 — scaffold (DONE)**: builds (x86_64 + aarch64 toolchain), installs,
  runs on emulator; permission-aware init + Resume hook; build script;
  manifest with permissions; this document.
- **M1 — metadata (DONE, 2026-10-27)**: `db.rs` + `scan.rs`; background scan
  thread → sqlite → `VecModel` in a `ListView` (paths as text). Acceptance
  verified on emulator: **5002 files in 153 ms**; relaunch `0+ 0~ 0-` in 2 ms;
  mtime-touch → `1~`, delete → `1-`. Status line shows `N files (a+ u~ d–) in X ms`.
  List paint is instant on relaunch: the DB list is shown first
  ("N files (cached)"), the background walk then runs and refreshes
  ("Updating… N"). First launch streams results into the list in 512-file
  batches while scanning — the UI never waits for the full DB build
  (`walk_stream`). Hidden dirs (`.thumbnails`, …) are skipped.
  Real-device numbers (Pixel-class, 8.5k photos): cold scan ~4.2–5.9 s
  (filesystem metadata cost — the walk is the bottleneck, not sqlite),
  relaunch paint instant, rescan ~4.2 s. Walk parallelism is an M2/M7
  optimization candidate.
  Gotchas hit (also see §6/§12):
  - `slint::Result` / `slint::WindowHandle` **do not exist in 1.18.1** as
    published on crates.io — desktop bin is `fn main() -> Result<(),
    slint::PlatformError>` + `use slint::ComponentHandle;`. (The M0 desktop
    bin never actually compiled; only the Android lib was built. The eval
    sample `slint_app` is broken the same way.)
  - `.slint` modulo is `Math.mod(x, y)`, not `%`.
  - ListView rows: `for path[index] in root.items: …` (`index` is not in
    scope unless named; `parent` = the ListView).
  - `rusqlite(bundled)` needs `CC_<triple>` pointing at NDK clang for
    Android — `scripts/build.sh` now exports them.
  - Android DB path: `EXTERNAL_DATA_DIR` is **not set** by this
    NativeActivity setup and `temp_dir()` = `/data/user/0` (unwritable).
    Use `/storage/emulated/0/Android/data/<pkg>/files`, internal files dir
    fallback (see `db::android_data_dir`).
  - Emulator: grant all-files access once via
    `adb shell appops set wow.gmp.gallery MANAGE_EXTERNAL_STORAGE allow`; it
    survives `adb install -r`.
  - Files pushed with `adb push` are owned by `shell` mode 640 — `chmod 644`
    or the app can't read them.
- **M2 — thumbnail engine (DONE, 2026-10-27)**: `images.rs` (turbojpeg
  DCT-level shrink decode via 1/2/1/4/1/8 `ScalingFactor`, all 8 EXIF
  orientations as uniform `(x,y)→(dx,dy)` pixel maps, `image::imageops::resize`
  final fit, re-encode to disk cache at q85; PNG via `image`) + `thumb.rs`
  (dispatcher thread round-robining to N=cores/2 workers clamped 2..8,
  unbounded job queue, disk cache `thumbs/{sha1(path|mtime|size|target)}.jpg`
  — target in the key so list-64px and grid-500px coexist; atomic
  `.tmp`→rename; generation counter for cancellation; per-worker sqlite
  connection writes `set_thumb` w/h/orientation/thumb_path) + `lib.rs`
  (process-wide `AppState` in `OnceLock`, 10 Hz drain timer — `Timer` is
  `!Send`, leaked with `std::mem::forget`, scroll-prefetch with 50-row
  margin, `do_prefetch` flag so streaming scan batches don't re-prefetch) +
  `main.slint` (rows carry `image` now).
  *Accept, verified:* 12 unit tests + 5k synthetic engine test (cold fill,
  100% cache warm refill, generation cancel). Release aarch64 APK 9.8 MB.
  **Real device (Pixel 9 Pro, 8577 photos):** cold scan ~5 s with list
  streaming; warm relaunch paints the cached list instantly; thumbnails
  render in every visible row (screenshot-verified); relaunch cache hits
  ~100% (255–258 of first 256 jobs); 2 WebP files misnamed `.jpg` fail
  gracefully (logged, row stays empty). Worker log cadence: every 256 jobs.
  *Known transient (unreproducible): the very first aarch64-release install
  rendered a pure-black screen (no drain-timer "painted" logs) until the
  next launch; it did **not** reproduce on the emulator (screen-off launch +
  wake) nor across repeated device launches. Suspected screen-lock / display
  state race on first GL frame. If it recurs: force a window refresh on
  Resume (see §10.7).*
  *Build gotchas hit (also in build.sh comments):*
  - `ANDROID_PLATFORM=android-26` (NDK CMake toolchain) **conflicts** with
    the `android-build` crate (Slint's Java compile) unless `ANDROID_JAR`
    is set explicitly to `platforms/android-35/android.jar`.
  - x86_64 host builds of turbojpeg-sys need `nasm` (`~/.local/bin/nasm`;
    aarch64 does not).
  - `ADB_SERIAL=…` env var picks the device in build.sh (phone + emulator
    are often both attached).
  - build.sh keywords: `debug` (x86_64), `debug-aarch64`, `x86_64`,
    `aarch64`; second arg `no-install` to skip install.
- **M3 — grid UI (DONE 2026-10-27)**: 3-column `ListView` grid (`GridRow` =
  one list item holding up to 3 square `CellView`s), shimmer placeholder via
  a time-driven `animation-tick()` expression, scroll-driven prefetch
  (`scrolled → on_scroll`), memory-bound retention (cells drop to shimmer
  when they leave the active range; disk cache repaints them in ~10 ms).
  Key fixes along the way (see §6.11–13): `in property` must be assigned
  explicitly in a `for`; cell updates must replace the whole outer row
  (`apply_row_batch`) and are capped at 16 rows/tick to avoid ANR on fast
  scroll; `on_scroll` only re-anchors the active range + submits prefetch
  (no model mutation), all mutation is in the 10 Hz drain timer.

  **Thumbnail loading: viewport reconciliation + worker-level priority
  (rewritten 2026-10-27 after two field-reported bugs).** Design doc is the
  header comment of `src/lib.rs`; engine in `src/thumb.rs`. The invariants:

  1. *The viewport is the only source of truth.* `on_scroll` only records the
     viewport range (zero work). The 10 Hz tick is the sole authority AND the
     sole submitter: CONSUME finished results (paint viewport ones, capped),
     EVICT far pixels (hard cap `MAX_PAINTED=120`), FILL the viewport
     (center-first), PRUNE queued jobs outside the retention band. After each
     tick: every viewport cell is PAINTED, IN-FLIGHT, or FAILED — restored,
     not maintained, so no scroll pattern can leave a visible cell forgotten
     (the original bug: re-submission only happened in `on_scroll`, so when
     scrolling STOPPED, forgotten cells stayed empty until the next small
     move). `debug_assert`ed every tick.
  2. *The viewport is first at EVERY queue level.* The central queue
     (`Mutex<VecDeque>` + `Condvar`, prepend-on-submit) is drained as
     priority-ordered bursts and PREPENDED to the per-worker queues (also
     `Mutex<VecDeque>` + `Condvar`, NOT mpsc — mpsc is FIFO-only). Each
     worker queue is capped (96 ≈ two screens): overflow is dropped from the
     TAIL (oldest transit work) with a `skipped: true` result; the UI clears
     the in-flight marker, the cell is NOT failed, re-entry re-submits it
     (disk-cache hit). A burst ≤ viewport (~45) < cap, so fresh viewport work
     is never dropped. `in_flight` accounting: +1 at submit, −1 at finish OR
     at cap-drop; the UI's `inflight` map (path→time) is the dedupe key and
     is cleared on every result (incl. `skipped`) — a dropped job whose
     path has a newer queued job may be briefly re-submitted (one wasted
     decode, self-cleans).

  Regression tests: `thumb::tests::scroll_back_cache_hit_not_starved`
  (central-level: 6.3 s FIFO starvation → 130 ms) and
  `thumb::tests::fresh_batch_preempts_stale_worker_queue` (worker-level: a
  fresh job submitted while a slow batch is queued must finish within the
  first two results, never behind the whole old queue).

  *Stress results (emulator x86_64 DEBUG build — slowest plausible hardware —
  then release aarch64 on the Pixel 9 Pro):* fast up/down ×5 (the reported
  repro): viewport fully covered 0–119 ms after stop (4/5 already covered at
  stop); cold landings: 1.0–1.2 s = pure decode throughput (p rises
  monotonically while stale jobs wait BEHIND, oldest growing 700→1900 ms
  harmlessly); chaos ×6 + 25 s idle soak: covered before every stop, max
  oldest in-flight job 217 ms (pre-fix: 8,055 ms), max queue depth 45 (one
  viewport), 0 invariant violations in ~1,450 ticks, 0 panics, idle
  converges to 0 queued. Phone smoke: 99.98% cache hits during 8k-cell
  stress scroll, 0 panics/ANRs, one 541 ms transient stall alarm (cold
  decode burst).

  *Verified (emulator, 150 real JPEGs + 5k stubs): first paint 0.6 s cold;
  warm relaunch from disk cache; 0 ANRs through 16 fast swipes; backlog
  drains to 0 when idle. Verified on Pixel 9 Pro (release aarch64, 8,577
  photos): first screen ~0.7 s cold at 500 px; scroll = sustained 60 fps
  with 0 display hitches (cold and warm cache) — see §8 for the
  measurement method (gfxinfo does NOT track Slint's window).*
- **M4 — viewer**: tap cell → full decode + overlay; pinch/pan (manual
  math), double-tap zoom; back gesture/button; EXIF orientation correct.
- **M5 — permissions**: implement `perm.rs` per §7; launch flow when
  un-granted (settings card → open Settings → recheck on Resume); test the
  exact passgo-style flow on a fresh install.
- **M6 — polish**: dark theme, animations (slide/fade), search/filter
  (name/date), sort; double-tap zoom; battery-friendly (pool idles, no
  busy repaint — Slint is retained, verify with gfxinfo "total frames").
- **M7 — perf & release**: real-device (aarch64) pass; Perfetto; memory
  bounds; release APK size check (expect ~8–9 MB like the sample);
  consider the 1.19 upgrade (verify pointer-scroll fix first).

## 10. Open questions & risks

1. **HEIC (RESOLVED for now)**: the real device has 0 HEIC files (9109
   JPEG, 27 PNG), so HEIC is deferred. When needed: add a decoder to
   `images.rs` dispatch (libheif-rs / `image` heic feature / libvips);
   the engine API is decoder-agnostic.
2. **Slint pointer-scroll `todo!()` panic** (open issue in the Android
   backend at eval time): could fire on trackpad/touch scroll. Test scrolling
   on M3; if it fires, patch locally (the fix is small) or upgrade to 1.19
   when available.
3. **Max texture size**: full-screen decode of 12 MP+ images (e.g.
   8192 px) may exceed GPU max texture size on some devices — verify on
   M4; fall back to a 2×-max downscale for display.
4. **Slint license**: royalty-free tier covers proprietary mobile/desktop/
   web; embedded devices excluded — irrelevant for us, but the repo README
   should state the chosen tier (royalty-free) once the app ships.
5. **Docs drift**: trust the pinned source (`~/prog/android/ui/slint-src`
   @ v1.18.1) over online docs; bump pin + re-verify after any Slint upgrade.
6. **rusqlite `bundled`** compiles SQLite from source in every target
   (adds ~30–60 s to clean builds) — acceptable; revisit if it hurts.
7. **Transient black screen (M2 first launch, unreproducible)**: one
   aarch64-release launch rendered pure black with a dead drain timer until
   relaunch; not reproduced on emulator or device since. Candidate fix if it
   recurs: in the `init_with_event_listener` Resume handler, call
   `window.request_redraw()` / check `window.size()` (a 0×0 or stale surface
   after a screen-off start would match the symptoms). Don't add it
   speculatively.

## 11. Pointers

- Framework eval (full report + raw notes + both sample apps + screenshots):
  `~/prog/android/ui/` — `REPORT.md`, `NOTES.md`, `slint_app/`, `eframe_app/`,
  `slint-src/` (v1.18.1), `egui-src/` (0.37.0).
- Slint docs (version-sensitive!): https://slint.dev/docs — Android guide:
  `docs/guide/platforms/mobile/android.mdx` in the source tree.
- `examples/native-gestures`, `demos/` in `slint-src` — reference code for
  gestures, lists, images.
- passgo (existing Go app using the same all-files-access pattern):
  `~/prog/go/passgo` — its `scripts/build.sh` and manifest handling are a
  proven reference for the permission UX flow.
- Emulator AVD: `pad_avd` (google_apis x86_64, API 35); start command in §3.
- Gitea: push target `git+ssh://gitea@git.wow.st/gmp/gallery.git`
  (repo must exist on the server first, like the other gmp/* repos).