Merge remote-tracking branch 'origin/rustify' into worktree-agent-a1ff0294b6c29127e

# Conflicts:
#	docs/RUST.md

docs/IRIS.md

@@ -8,6 +8,37 @@ capability that moved. Small and trivial changes do not go here.
 An entry gives the date, what changed, why, and a short before/after where
 it helps judge the change without the session that made it. Newest first.
 
+## 2026-09-06: `List::fling`, `VelocityTracker`, `FlingCalculator` (IRIS_TODO.md's "swiping has no momentum")
+
+`iris::widget::List` gained a real fling: `fling(velocity_px_per_s)` starts
+one (cancelled by the next touch-down via `cancel_fling`, or automatically
+once it settles or reaches loaded content's start/end), `is_scrolling()`
+reports whether one is running, and `tick_fling(now: Instant) -> bool`
+advances it and returns whether it is still going -- a caller that owns a
+`RequestRedraw` handle can hand it to the list once via the new
+`set_redraw_handle`, after which `List` re-arms its own next frame while
+flinging with no further polling needed; a caller driving a scripted
+benchmark instead calls `tick_fling` itself in a loop, same as it already
+drives `scroll`.
+
+The physics is `iris::sense::FlingCalculator` + `VelocityTracker`
+(`sense.rs`, beside `DragArbiter`): a port of AOSP `SplineOverScroller`'s
+deceleration curve (the same one Compose's own `ScrollableDefaults.
+flingBehavior()` uses), cited at the definition, so a fling here travels
+the same distance a Compose `LazyColumn` would for the same initial
+velocity. `VelocityTracker` estimates that velocity from the drag's last
+~100ms of samples rather than one frame's last delta. Unit-tested:
+velocity from known samples, fling distance/duration against the closed-
+form spline result (within 1%), cancel-on-touch, and the start/end clamp
+(a fling stops rather than scrolling into content that was never loaded).
+
+Before: a touch-drag panned exactly as far as the finger moved and stopped
+dead on release. After: releasing mid-drag continues scrolling and
+decelerates, matching the muscle memory every other Android scroll view
+already trained. `transcript_ui::selection::Selection::drag` wires this in
+-- a release only flings if the gesture had committed to panning
+(`DragArbiter::is_panning`, new), never a selection or an undecided tap.
+
 ## 2026-09-06: `UiRenderNode::new` returns `Result`, not `Self` (RUST.md's P0 box, phone-crash fix)
 
 `iris_core::UiRenderNode::new(device, queue, config)` now returns

docs/IRIS_TODO.md

@@ -124,20 +124,31 @@ follow-ups. Recorded here rather than fixed in that pass, so a follow-up
 agent takes them without colliding with that pass's `bench_client.rs`/
 `android/view.rs`/`android/sense.rs` changes.
 
-- [ ] **Swiping has no momentum.** iris's `List` pans exactly as far as the
-  finger moves and stops dead on release -- unlike Compose, which flings
-  and decelerates. Needs velocity tracking over the last several move
-  samples and an Android-style decelerating fling, cancelled by the next
-  touch-down, redrawing every frame until it settles. `BenchRun.kt`'s own
-  fling phase (RUST.md's "Benchmark v2" box) is the reference shape:
-  "travel way faster... which is better for stress testing."
-- [ ] **Scrolling down sometimes jitters the text.** Two named suspects,
-  neither confirmed: `DragArbiter`'s slop being released as one jump
-  (`iris/src/sense.rs`), or a per-frame pan delta applied a frame late.
-  Measure by tracing the list's scroll offset per frame against a
-  monotonic synthetic drag, the same way `iris/android-app/trace-draw.sh`-
-  style instrumentation traced the touch-scroll dropout in RUST.md's I5
-  box -- not by eyeballing a screenshot.
+- [x] **Swiping has no momentum, fixed 2026-09-06.** `List::fling`/
+  `VelocityTracker`/`FlingCalculator` (`iris/src/widget/list.rs`,
+  `iris/src/sense.rs`) -- IRIS.md's 2026-09-06 entry has the full account.
+  Wired through `Selection::drag`'s release path, cancelled by the next
+  touch-down, clamped at the loaded content's start/end. Verified by unit
+  test (fling distance against the closed-form spline result, cancel-on-
+  touch, the clamp), not yet by an on-device or emulator feel-check --
+  that is still open.
+- [x] **Scrolling down sometimes jitters the text, fixed 2026-09-06.**
+  Root-caused by reading `DragArbiter::update`'s `Undecided`-to-`Panning`
+  transition rather than by an on-device trace (no emulator was used this
+  pass): it was the first named suspect, not the second. `self.last` stays
+  at the press origin for every `Undecided` frame (nothing pans while the
+  gesture might still be a selection), so the frame that finally crosses
+  `DRAG_SLOP` returned `Pan(dy)` with `dy` measured from `press_start` --
+  the *whole* pre-threshold drag, applied to the list in one step, however
+  many frames it had taken to get there. Fixed by applying only the
+  excess past `DRAG_SLOP` on that one frame (`dy - DRAG_SLOP.copysign
+  (dy)`), the same "consume the slop, don't replay it" rule Android's own
+  touch handling follows. New regression test,
+  `crossing_the_slop_by_a_little_pans_by_a_little` (`iris/src/sense.rs`).
+  **Not yet done**: an emulator trace of the real per-frame offset
+  confirming this was the whole story on real touch input rather than
+  only the arbiter's own unit tests -- worth a follow-up pass before
+  calling it fully closed.
 
 ## Build
 

docs/RUST.md

@@ -4561,6 +4561,26 @@ device.
       two-density crispness check IRIS_TODO.md's unit item asks for, and
       the two open items just above.
 
+      **Fling and jitter, 2026-09-06.** The two `IRIS_TODO.md` "From the
+      phone" items this box's own text names as follow-ups are fixed --
+      `List::fling`/`VelocityTracker`/`FlingCalculator` (IRIS.md's
+      2026-09-06 entry) and the `DragArbiter` slop-release jump (fixed by
+      applying only the excess past `DRAG_SLOP` on the crossing frame,
+      not the whole pre-threshold drag) -- both wired through
+      `Selection::drag`'s release path, both covered by new unit tests in
+      `iris/src/sense.rs` and `iris/src/widget/list.rs`. **Root-caused by
+      reading `DragArbiter::update` and testing it directly, not by an
+      emulator trace** -- this pass did not open an emulator, so the
+      "trace the list's offset per frame" verification this box's own
+      todo asked for is still open, as is a feel-check of the fling on
+      real touch input. **Benchmark v2's four-phase spec (fling/stream/
+      type/keyboard) in `bench_client.rs` was not attempted this pass** --
+      wiring a real IME show/hide and refresh-rate read through
+      `bench_jni.rs`, and `FrameReport`'s per-phase accounting, is real
+      scope on its own and was left rather than shipped half-verified;
+      the Compose half above is already done and is the reference shape
+      for whoever picks this up. No redelivery this pass.
+
 - [ ] **P1 — session screen parity.** History paging backward (with the
       page-boundary healing `client-core` does not have yet, below),
       `TranscriptSource`-backed cache/server stitching, jump-to-latest,