iris: a scroll area no longer opens at the end of content it has not measured

Scroll::content_len was 0.0 both for 'nothing here' and for 'not drawn
yet', so the first frame's clamp found a scroll range of zero, read
amt == len as 'sitting at the end' and set snap_end -- and the next
frame, now knowing the real length, jumped to it. On a phone that put a
code fence at the end of its longest line, mid-word, before anybody
touched it. It is an Option now, and the clamp does not answer a question
it cannot yet answer.

Which edge an area opens at is also a caller's decision rather than a
default: scrollable_on starts at the beginning (what is read),
scrollable_to_end pins to the end while content grows (what is typed --
the composer), both through one Scroll::new(inner, axis, at_end).

And tool.rs's raw_block pans sideways again: the 2026-09-06 'a
scrollable_on(Axis::X) around a non-editable Text draws nothing' defect
does not reproduce, most likely fixed by the shaped-mask work, so a long
command is readable rather than clipped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/IRIS_TODO.md

@@ -801,18 +801,32 @@ has the fuller account.
   `reposition` fault f5b8893 fixed -- it survives that commit. The
   workaround costs the group the 4dp inset its Compose counterpart holds
   its cards off the edge by, so this is worth fixing.
-- [ ] **`scrollable_on(Axis::X)` on a non-editable `Text` draws nothing.**
+- [x] **`scrollable_on(Axis::X)` on a non-editable `Text` draws nothing.**
   The panel is drawn and the text inside it is not. A markdown fence does
   the same to a `TextEdit` and is fine, so it is the widget kind rather
   than the chain. `tool.rs`'s `raw_block` is `masked()` only until this is
   fixed, which means a long command is clipped rather than pannable.
+  **Not reproducible on 2026-09-08**: `raw_block` was changed to
+  `.scrollable_on(Axis::X).pad(..).masked_by(..)` and the command draws
+  normally (`IRIS_TOOLS_EXPANDED=1 iris/run-headless.sh transcript --shot`,
+  the `rm -rf target` card). Something between 09-06 and 09-08 fixed it --
+  the shaped-mask work (`.masked_by`, 38bf630) is the likeliest, since the
+  old chain was `.masked()` *inside* the padding. Left ticked with the
+  original symptom recorded rather than deleted, in case it comes back.
 - [ ] **No overflow ellipsis.** `TextAttrs` can wrap or not wrap; there is
   no "one line, ellipsised" the way `maxLines = 1` + `TextOverflow.
   Ellipsis` gives Compose. A tool card's summary is clipped instead, so
   nothing on screen says it was cut. Whichever end is cut has to be a
   choice when this lands: a path is identified by its tail, a command by
   its head.
-- [ ] **A drawn chevron.** `Chevron.kt` draws its own strokes precisely
+- [ ] **A drawn chevron.** *Still open, and now worse: the bundled fonts
+  were removed on 2026-09-07 in favour of the platform collection, so the
+  mark is a codepoint the phone's own faces may not have -- Iris's
+  2026-09-08 screenshot shows an empty box where it should be, and the
+  desktop render draws it as a small dot. UI_RULES: "don't rely on
+  characters the platform might not have." iris has rects, text and
+  textures, so the smallest honest fix is a generated texture rather than
+  a new primitive.* `Chevron.kt` draws its own strokes precisely
   because a chevron from a font is a glyph a system font may not have --
   and the bundled `NotoSans-Regular.ttf` indeed has no U+25B8/25BE/25B4,
   while `NotoSansMono-Regular.ttf` does. `tool.rs` sets the mark in the

iris/examples/bench_images.rs

@@ -51,7 +51,10 @@ impl DefaultAppState for State {
         }
         let span = rsc.ui.widgets.add_strong(span);
         let span_weak = span.weak();
-        let root = rsc.ui.widgets.add_strong(Scroll::new(span.any(), Axis::Y));
+        let root = rsc
+            .ui
+            .widgets
+            .add_strong(Scroll::new(span.any(), Axis::Y, true));
         ui_state.set_root(root.any());
         Self {
             ui_state,

iris/src/layout_tests.rs

@@ -50,7 +50,10 @@ fn scrolled_rects(
         span.push(row.any());
     }
     let span = rsc.ui.widgets.add_strong(span);
-    let scroll = rsc.ui.widgets.add_strong(Scroll::new(span.any(), Axis::Y));
+    let scroll = rsc
+        .ui
+        .widgets
+        .add_strong(Scroll::new(span.any(), Axis::Y, true));
     let weak = scroll.weak();
     (weak, scroll.any(), rects)
 }
@@ -324,7 +327,10 @@ fn a_scroll_measures_the_box_it_was_offered_not_the_window() {
         x: None,
         y: Some(Len::abs(1000.0)),
     });
-    let scroll = rsc.ui.widgets.add_strong(Scroll::new(tall.any(), Axis::Y));
+    let scroll = rsc
+        .ui
+        .widgets
+        .add_strong(Scroll::new(tall.any(), Axis::Y, true));
     let scroll_w = scroll.weak();
     let scroll_id = scroll.id();
     let capped = rsc.ui.widgets.add_strong(MaxSize {
@@ -387,7 +393,10 @@ fn a_panned_widgets_own_hit_box_moves_exactly_once() {
         y: Some(Len::abs(1000.0)),
     });
     let tall_w = tall.weak();
-    let scroll = rsc.ui.widgets.add_strong(Scroll::new(tall.any(), Axis::Y));
+    let scroll = rsc
+        .ui
+        .widgets
+        .add_strong(Scroll::new(tall.any(), Axis::Y, true));
     let scroll_w = scroll.weak();
     let root = scroll.any();
 
@@ -907,3 +916,50 @@ fn a_plain_mask_still_clips_to_a_square_box() {
         "a square clip must reject a point outside it",
     );
 }
+
+/// A scroll area created to be *read* opens at the beginning of its
+/// content, however many frames it takes to learn how long that content
+/// is.
+///
+/// The bug this pins: `content_len` was `0.0` both for "nothing here" and
+/// for "not drawn yet", so the first frame's clamp found a range of zero,
+/// read `amt == len` as "sitting at the end", and set `snap_end` -- and
+/// the frame after, now knowing the real length, jumped to it. On screen
+/// that was a code fence opening at the end of its longest line, in the
+/// middle of a word (`iris/run-headless.sh phone`, 2026-09-08).
+#[test]
+fn a_scroll_area_opens_at_the_start_of_content_it_has_not_measured_yet() {
+    for (name, at_end, want) in [("read", false, 0.0), ("written", true, 4900.0)] {
+        let mut rsc = TestRsc {
+            ui: UiData::default(),
+        };
+        let fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE)).any();
+        let tall = rsc.ui.widgets.add_strong(Sized {
+            inner: fill,
+            x: None,
+            y: Some(Len::abs(5000.0)),
+        });
+        let scroll = rsc
+            .ui
+            .widgets
+            .add_strong(Scroll::new(tall.any(), Axis::Y, at_end));
+        let weak = scroll.weak();
+        let root = scroll.any();
+
+        let mut render = UiRenderState::new();
+        render.resize((800.0, 100.0));
+        // Twice: the first draw is the one that measures the content, and
+        // the defect only showed on the second. The touch in between is
+        // what asks for that second draw -- an unchanged frame draws
+        // nothing at all, which is the point of the frame before it.
+        render.update(&root, &mut rsc);
+        let _ = rsc.ui.widgets.get_mut(&weak);
+        render.update(&root, &mut rsc);
+
+        let amt = rsc.ui.widgets.get(&weak).unwrap().amt();
+        assert!(
+            (amt - want).abs() < 0.01,
+            "an area to be {name} should have opened at {want}, got {amt}"
+        );
+    }
+}

iris/src/widget/position/scroll.rs

@@ -8,7 +8,18 @@ pub struct Scroll {
     amt: f32,
     snap_end: bool,
     container_len: f32,
-    content_len: f32,
+    /// How long the content is along `axis`, as of the last draw --
+    /// `None` until this widget has drawn once.
+    ///
+    /// An `Option` rather than a `0.0` that stands in for both, because
+    /// the two answers led somewhere different and the code could not
+    /// tell them apart: on the first frame the clamp in `update_amt`
+    /// computed a scroll range of zero, concluded from `amt == len` that
+    /// the area was sitting at its end, and set `snap_end` -- so the next
+    /// frame, now knowing the real length, jumped to it. A code fence
+    /// therefore opened at the end of its longest line, mid-word
+    /// (`iris/run-headless.sh phone`, 2026-09-08).
+    content_len: Option<f32>,
     /// Touch panning, from the same `DragGesture` `List` is driven by
     /// (`transcript-ui::Selection::drag`) rather than a second copy of its
     /// wiring: arbitration, `DRAG_SLOP` and pointer capture all live in
@@ -48,13 +59,19 @@ impl Widget for Scroll {
         let container_len = painter.px_size().axis(axis);
         self.container_len = container_len;
 
-        if self.snap_end {
-            self.amt = self.content_len - self.container_len;
+        if self.snap_end
+            && let Some(content_len) = self.content_len
+        {
+            self.amt = content_len - self.container_len;
         }
         self.update_amt();
 
         let mut region = UiRegion::FULL;
-        region.axis_mut(axis).end = region.axis(axis).start.offset(self.content_len);
+        // The container's own length until the content has been measured:
+        // a zero-length region on the first frame would place the child's
+        // primitives against a box of no size.
+        let offered = self.content_len.unwrap_or(container_len);
+        region.axis_mut(axis).end = region.axis(axis).start.offset(offered);
         let region = region.offset(Vec2::from_axis(axis, -self.amt, 0.0));
 
         let used = painter.widget_within(&self.inner, region);
@@ -62,10 +79,11 @@ impl Widget for Scroll {
         // A child reporting `rel` means "this fraction of what I was
         // offered", and what it was offered is this scroll area -- so the
         // container, again, is what that resolves against.
-        self.content_len = used
-            .axis(axis)
+        self.content_len = Some(
+            used.axis(axis)
                 .apply_rest(painter.density())
-            .to_abs(container_len);
+                .to_abs(container_len),
+        );
 
         // The **content's** size, not the container's. A parent that can
         // grow (the composer's bar) should hug the text until its own cap
@@ -80,14 +98,18 @@ impl Widget for Scroll {
 }
 
 impl Scroll {
-    pub fn new(inner: StrongWidget, axis: Axis) -> Self {
+    /// `at_end` starts the area pinned to the end of its content and
+    /// keeps it there while the content grows -- see
+    /// `WidgetLike::scrollable_to_end`. `false` starts at the beginning,
+    /// which is what anything being *read* wants.
+    pub fn new(inner: StrongWidget, axis: Axis, at_end: bool) -> Self {
         Self {
             inner,
             axis,
             amt: 0.0,
-            snap_end: true,
+            snap_end: at_end,
             container_len: 0.0,
-            content_len: 0.0,
+            content_len: None,
             gesture: DragGesture::on(axis),
         }
     }
@@ -148,10 +170,19 @@ impl Scroll {
         }
     }
 
+    /// Clamp `amt` into the range the content allows, and re-read whether
+    /// this area is sitting at its end.
+    ///
+    /// Both are skipped until the content has been measured: with no
+    /// length there is no range to clamp into, and "at the end" is a
+    /// question that cannot be answered yet -- answering it anyway is
+    /// what `content_len`'s doc describes.
     pub fn update_amt(&mut self) {
-        self.amt = self.amt.max(0.0);
-        let len = (self.content_len - self.container_len).max(0.0);
-        self.amt = self.amt.min(len);
+        let Some(content_len) = self.content_len else {
+            return;
+        };
+        let len = (content_len - self.container_len).max(0.0);
+        self.amt = self.amt.clamp(0.0, len);
         self.snap_end = self.amt == len;
     }
 
@@ -182,8 +213,8 @@ mod tests {
         let mut ui = UiData::default();
         let inner = ui.widgets.add_strong(Rect::new(UiColor::WHITE)).any();
         let id = inner.id();
-        let mut s = Scroll::new(inner, Axis::Y);
-        s.content_len = 1000.0;
+        let mut s = Scroll::new(inner, Axis::Y, true);
+        s.content_len = Some(1000.0);
         s.container_len = 100.0;
         s.amt = 400.0;
         s.snap_end = false;

iris/src/widget/trait_fns.rs

@@ -87,13 +87,34 @@ widget_trait! {
         self.scrollable_on(Axis::Y)
     }
 
+    /// `scrollable_on`, but starting pinned to the **end** of its content
+    /// and staying there while the content grows -- what a composer wants,
+    /// where the newest line is the one being written.
+    ///
+    /// Explicit, because the other kind is not a variation on it: a code
+    /// fence opened at the end of its longest line, which is the middle
+    /// of a word (seen in `iris/run-headless.sh phone`, 2026-09-08). The
+    /// two behaviours are one mechanism with the starting edge passed in,
+    /// and both names say which they are rather than one of them being a
+    /// default nobody reads.
+    fn scrollable_to_end(self, axis: Axis) -> impl WidgetIdFn<Rsc, Scroll> where Rsc: HasEvents {
+        self.scroll_area(axis, true)
+    }
+
     /// `scrollable` along `axis`. A code fence pans across its own long
     /// lines exactly the way a transcript pans down its rows, so the two
     /// are one function with the axis passed in rather than a second copy
     /// -- `DragArbiter::on` is the other half.
     fn scrollable_on(self, axis: Axis) -> impl WidgetIdFn<Rsc, Scroll> where Rsc: HasEvents {
+        self.scroll_area(axis, false)
+    }
+
+    /// The one implementation behind [`Self::scrollable_on`] and
+    /// [`Self::scrollable_to_end`] -- see the latter for what `at_end`
+    /// decides.
+    fn scroll_area(self, axis: Axis, at_end: bool) -> impl WidgetIdFn<Rsc, Scroll> where Rsc: HasEvents {
         move |state| {
-            Scroll::new(self.add_strong(state), axis)
+            Scroll::new(self.add_strong(state), axis, at_end)
                 .on(CursorSense::Scroll, move |ctx, rsc| {
                     let delta = ctx.data.scroll_delta.axis(axis) * 50.0;
                     ctx.widget(rsc).scroll(delta);

iris/transcript-ui/src/composer.rs

@@ -100,7 +100,9 @@ where
     // transcript: measured before this change at 58px of stray text for a
     // 475px message in a 417px box.
     let content = field
-        .scrollable()
+        // `scrollable_to_end`: what is being typed is at the end, so a
+        // message longer than the six lines shown holds that end.
+        .scrollable_to_end(Axis::Y)
         .masked()
         .pad(dp(FIELD_PAD_DP))
         .max_height(dp(APPROX_LINE_HEIGHT_DP * MAX_LINES + FIELD_PAD_DP * 2.0))

iris/transcript-ui/src/tool.rs

@@ -272,9 +272,9 @@ where
         .wrap(false)
         .add(rsc);
     field
-        .masked()
+        .scrollable_on(Axis::X)
         .pad(dp(RAW_PAD_DP))
-        .background(rect(VERBATIM_BACKGROUND).radius(dp(RAW_RADIUS_DP)))
+        .masked_by(rect(VERBATIM_BACKGROUND).radius(dp(RAW_RADIUS_DP)))
         .width(rest(1))
         .add_strong(rsc)
         .any()