iris: DragArbiter closes I5's touch-drag pan-vs-select gap

A row's own click_or_drag() selection handler always won the same
gesture a list-level pan wanted, since run_sensors gives the inner
layer first refusal every frame it's pressed. DragArbiter
(iris/src/sense.rs) decides pan vs. select the way Android does:
vertical drag pans immediately, a held stationary press starts a
selection after LONG_PRESS, and a horizontal drag on already-selected
text extends immediately. transcript-ui's Selection::drag routes
every row's drag through one arbiter per list, driving List::scroll
for a pan instead of a second scroll mechanism.

8 new unit tests (iris::sense::drag_arbiter_tests); cargo
fmt/clippy/test --workspace and cargo ndk (iris, transcript-ui) all
clean; run-headless.sh screenshot byte-identical to before the change.

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

iris/transcript-ui/src/row.rs

@@ -20,7 +20,7 @@ use crate::markdown::render_markdown;
 use crate::selection::Selection;
 use client_core::transcript_fold::{QuestionCard, TranscriptItem, TranscriptRow as FoldedRow};
 use iris::prelude::*;
-use std::{cell::RefCell, rc::Rc};
+use std::{cell::RefCell, rc::Rc, time::Instant};
 
 /// The paragraph size every row's `TextEdit` is built at; markdown headings
 /// inside a row scale relative to a fixed set of sizes rather than this one
@@ -100,11 +100,13 @@ fn tool_call_markdown(tool: &str, input: &str, output: &str) -> String {
 
 /// Build one `TextEdit` from a sender label plus markdown source, register
 /// it with `selection` under `key`, and wire the pointer handlers that
-/// drive `Selection::begin`/`extend` -- shared by every row variant below,
-/// since a selectable row is always "one TextEdit plus this wiring"
-/// regardless of what folded it.
+/// drive `Selection::drag` -- shared by every row variant below, since a
+/// selectable row is always "one TextEdit plus this wiring" regardless of
+/// what folded it. `list` is threaded through so that same drag can pan
+/// the list instead of selecting, per `Selection::drag`'s own doc.
 fn build_text_row<Rsc: HasEvents>(
     rsc: &mut Rsc,
+    list: WeakWidget<List>,
     selection: Rc<RefCell<Selection>>,
     key: RowKey,
     sender: Option<&str>,
@@ -125,18 +127,24 @@ where
     selection.borrow_mut().register(key, field);
 
     field
-        .on(CursorSense::click_or_drag(), move |ctx, rsc| {
-            let sel = selection.clone();
-            match ctx.data.sense {
-                CursorSense::PressStart(_) => {
-                    sel.borrow_mut()
-                        .begin(rsc, key, ctx.data.pos, ctx.data.size)
-                }
-                _ => sel
-                    .borrow_mut()
-                    .extend(rsc, key, ctx.data.pos, ctx.data.size),
-            }
-        })
+        // `| CursorSense::unclick()` on top of the usual click-or-drag set
+        // -- the arbiter inside `Selection::drag` needs the release too,
+        // to go back to idle for the next press (`DragArbiter::release`).
+        .on(
+            CursorSense::click_or_drag() | CursorSense::unclick(),
+            move |ctx, rsc| {
+                selection.borrow_mut().drag(
+                    rsc,
+                    list,
+                    key,
+                    ctx.data.pos,
+                    ctx.data.size,
+                    ctx.data.cursor.pos,
+                    ctx.data.sense,
+                    Instant::now(),
+                );
+            },
+        )
         .add(rsc);
 
     // `.add` (weak), not `.add_strong` -- `header` is about to be embedded
@@ -165,6 +173,7 @@ where
 
 fn build_single<Rsc: HasEvents>(
     rsc: &mut Rsc,
+    list: WeakWidget<List>,
     selection: Rc<RefCell<Selection>>,
     key: RowKey,
     item: &TranscriptItem,
@@ -173,7 +182,7 @@ where
     Rsc::State: FocusHost,
 {
     let (sender, markdown_src) = item_content(item);
-    build_text_row(rsc, selection, key, sender, &markdown_src)
+    build_text_row(rsc, list, selection, key, sender, &markdown_src)
 }
 
 /// A run of adjacent tool calls: collapsed to a one-line summary by
@@ -215,6 +224,7 @@ where
 
     fn build_content<Rsc: HasEvents>(
         rsc: &mut Rsc,
+        list: WeakWidget<List>,
         selection: Rc<RefCell<Selection>>,
         key: RowKey,
         expanded: bool,
@@ -225,11 +235,12 @@ where
         Rsc::State: FocusHost,
     {
         let text = if expanded { full } else { summary };
-        build_text_row(rsc, selection, key, Some("Tools"), text)
+        build_text_row(rsc, list, selection, key, Some("Tools"), text)
     }
 
     let content = build_content(
         rsc,
+        list,
         selection.clone(),
         key,
         false,
@@ -252,6 +263,7 @@ where
         *expanded.borrow_mut() = !was_expanded;
         let content = build_content(
             rsc,
+            list,
             selection.clone(),
             key,
             !was_expanded,
@@ -279,7 +291,7 @@ where
     match row {
         FoldedRow::Single(item) => {
             let key = row_key(&item.key());
-            (key, build_single(rsc, selection, key, item))
+            (key, build_single(rsc, list, selection, key, item))
         }
         FoldedRow::Tools(calls) => {
             let key = row_key(&calls[0].key());