iris: the composer scrolls on a finger -- a dp cap worth zero, a stale mask slot, a hit box moved twice

Wrapping the composer's field in .scrollable().masked() needed three
layout defects fixed first, each with a headless regression test that was
confirmed to fail without its fix:

- MaxSize/Sized reported a caller's declared dp length unresolved, and
  Span places a child from the abs/rel of what it reported, so dp(168)
  was worth zero: the bar got a slot of nothing the moment its content
  passed six lines and the Scroll inside measured its container at -63px
  (container=-63 content=415.8 amt=478.8 on the emulator). Len::fold_dp,
  used on the way out, plus a debug_assert in draw_inner that a reported
  Size carries no dp -- the rule is about every widget, not those two.
- Masked allocated a fresh mask slot per draw, and draw_inner's
  unchanged-region fast path does not revisit descendants, so they kept
  clipping against a box the bar had moved away from: four live mask
  entries, none of them current, and the field drew nothing.
  ActiveData::own_mask, allocated once and rewritten in place.
- mov updates active.region and accumulates the same delta on the move
  slot, and resolved_region added both, so a panned widget's own hit box
  sat at twice the pan -- the composer's field was untappable after a
  drag. ActiveData::move_applied.

Scroll itself measured the right number by a misleading route; it is
written against painter.px_size() now and still reports its content's
size, since reporting the container makes the answer a function of
itself.

Verified on this checkout's emulator: swipe 540 1200 -> 540 1460 moved
the field's Message box 31,1041..1048,1509 -> 31,1131..1048,1651 with its
height unchanged at 468px.

run-bench.sh polled logcat for a prefix copy_report also logs at startup,
so it printed a report that had never been run.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/RUST.md

@@ -43,6 +43,83 @@ gated on her verdict**, so this pass works the P0 defects and the pure
 prerequisites in this order. Each item is ticked here by the agent that
 closes it.
 
+### Task A, closed 2026-09-06: the composer scrolls on a finger
+
+`iris/transcript-ui/src/composer.rs` is `field.scrollable().masked()` now.
+Verified on this checkout's emulator -- the evidence and the numbers are
+in docs/IRIS_TODO.md's ticked "composer has no touch-drag scroll" item.
+
+**The premise the task was given under was wrong, and that is worth
+recording**: `Scroll` did *not* measure against the window. Its
+`used.within_len(container).to_abs(output_size)` came to exactly
+`abs + rel * container_px` -- the right number by a route that reads as if
+the window were the container, which is what cost a session. It is
+`painter.px_size()` and `to_abs(container_len)` now: same arithmetic,
+stated the way the invariant is. `Scroll` also still reports its
+**content's** size upward, deliberately -- reporting the container makes
+the answer a function of itself (the bar is sized *from* that report, so
+it collapses to nothing and never recovers; measured in the headless
+harness before the shape was settled).
+
+What actually broke the composer was three separate defects, each now
+carrying a headless regression test in `iris/src/layout_tests.rs` that was
+confirmed to fail without its fix:
+
+1. **A `MaxSize` reported its cap as an unresolved `dp`.**
+   `Span::draw` places a child from the `abs`/`rel` of the length it
+   reported, so `dp(168)` was worth **zero** and the bar got a slot of
+   nothing the instant its content passed six lines; the `Scroll` inside
+   then measured its container at **-63px** (the padding subtracted from
+   nothing) and panned the whole message out of view. Emulator log, before
+   the fix: `container=-63 content=415.8 amt=478.8`. Fixed by
+   `Len::fold_dp` (new), used by `MaxSize` and `Sized` on the way out, and
+   guarded for every widget by a `debug_assert!` in
+   `UiRenderState::draw_inner` that a reported `Size` carries no `dp`.
+   Test: `a_dp_cap_is_reported_in_pixels_so_a_span_can_place_it`.
+2. **A `Masked` allocated a fresh mask slot on every draw.**
+   `draw_inner`'s unchanged-region fast path means its descendants are
+   mostly *not* redrawn with it, so they kept clipping against the slot
+   they were first drawn under -- measured on the composer's tree at
+   **four live mask entries, none of them the widget's current box**, and
+   the field drew nothing at all. The slot is allocated once and rewritten
+   in place now (`ActiveData::own_mask`, `Painter::set_mask`), with its
+   path out in `remove`'s `undraw` branch. Test:
+   `a_masked_widget_keeps_one_mask_slot_that_is_always_its_own_region`.
+3. **A panned widget's own hit box moved twice.** `mov` updates
+   `active.region` *and* accumulates the same delta on the widget's move
+   slot, and `resolved_region` added both -- so after a finger pan the
+   composer's field was untappable, while its descendants were fine (which
+   is why `hit_testing_follows_a_scrolled_widget`, which checks a
+   descendant, never saw it). `ActiveData::move_applied` records the part
+   of the slot's delta `region` already accounts for. Test:
+   `a_panned_widgets_own_hit_box_moves_exactly_once` (fails at exactly
+   2x the pan without it).
+
+Still open, and **pre-existing** (present in the build before this change,
+so not the scroll area's doing): the composer bar's grey background is not
+drawn on the `transcript-screen bench` build, so the message reads as
+white text over the transcript. `Stack{StackSize::Child(1)}` is the thing
+to look at.
+
+Rig fix on the way past: `iris/android-app/run-bench.sh` polled logcat for
+`"iris bench report:"`, which `copy_report` also logs at startup
+("nothing to copy -- run the benchmark first"), so it returned instantly
+and printed a report that had never been run. It polls for the report's
+own first line now.
+
+### Bench, before Task B (emulator, 2026-09-06)
+
+`iris/android-app/build-apk.sh debug --abi x86_64 --features
+"transcript-screen bench force-gles"` + `run-bench.sh`, this checkout's
+AVD. Emulator absolutes transfer nothing; the before/after ratio on the
+same emulator does.
+
+    stream: 202 frames over 21.0s
+      late: 197 (97.5%)
+        total  p50 61.5ms  p90 211.7ms  p99 342.6ms
+      worst 403.6ms
+
+
 - [x] **Merge the `DragGesture` work** -- done 2026-09-06 (merge commit
       `f802de9`, `git merge --no-ff worktree-agent-a754368325fa06839`,
       clean, no conflicts across the 8 files `e12c708` touched). Targets