Link the ordinary tests once, and keep their debug info to line tables

Eleven `tests/*.rs` were eleven binaries, each linking the whole graph --
`wgpu` and all -- to run a handful of cases. They are modules of one target
now, under `tests/cases/`, and `cargo test --test suite layout::` still picks
one out. The fuzzers and the `*_cost` measurements stay their own targets:
they are run on their own and want to be selectable without building the
rest.

`profile.test` takes `debug = "line-tables-only"`, which is what a backtrace
here actually reads; the type and variable information was the bulk of what
the linker was writing.

Measured on this machine, rebuilding `iris`'s test targets after a change to
the crate: 14.3 s before, 9.8 s with one target, 7.7 s with both. `target/`
went from 45 GB to 13 GB. The suite still passes 102 tests, and the binary
still carries `.debug_line`.

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

tests/cases/pointer_routing.rs

@@ -0,0 +1,225 @@
+//! Input across layers: what stops at a layer, what passes through it, and
+//! where hovering stops.
+
+use std::{cell::RefCell, rc::Rc};
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+const WINDOW: f32 = 100.0;
+
+/// Every sense that has fired on one widget since it was last read.
+#[derive(Default, Clone)]
+struct Fired(Rc<RefCell<Vec<CursorSense>>>);
+
+impl Fired {
+    fn take(&self) -> Vec<CursorSense> {
+        std::mem::take(&mut self.0.borrow_mut())
+    }
+}
+
+/// A widget filling whatever it is given, recording the senses it is sent.
+fn listener(h: &mut Harness, senses: impl Into<CursorSenses>) -> (WeakWidget<Rect>, Fired) {
+    let fired = Fired::default();
+    let record = fired.clone();
+    let id = rect(Color::WHITE)
+        .on(senses.into(), move |ctx, _| {
+            record.0.borrow_mut().push(ctx.data.sense)
+        })
+        .add(&mut h.rsc);
+    (id, fired)
+}
+
+/// A widget with no senses of its own, to leave a gap beside one that has.
+fn blank(h: &mut Harness) -> WeakWidget<Rect> {
+    rect(Color::WHITE).add(&mut h.rsc)
+}
+
+fn harness() -> Harness {
+    Harness::new((WINDOW, WINDOW))
+}
+
+#[test]
+fn hover_stops_at_the_topmost_widget() {
+    let mut h = harness();
+    let (bottom, bottom_hover) = listener(&mut h, CursorSense::HoverStart);
+    let (middle, middle_hover) = listener(&mut h, CursorSense::HoverStart);
+    let (top, top_hover) = listener(&mut h, CursorSense::HoverStart);
+    h.set_root((bottom, middle, top).stack());
+
+    h.move_to((50, 50));
+
+    assert_eq!(top_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        middle_hover.take(),
+        [],
+        "hover is not shared with a layer below"
+    );
+    assert_eq!(bottom_hover.take(), []);
+}
+
+#[test]
+fn a_scroll_passes_through_every_widget_that_does_not_want_it() {
+    let mut h = harness();
+    let (list, scrolled) = listener(&mut h, CursorSense::Scroll);
+    let (button, clicked) = listener(&mut h, CursorSense::click());
+    let (overlay, overlay_clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((list, button, overlay).stack());
+
+    h.move_to((50, 50));
+    h.scroll((0, 10));
+
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "two layers of click-only widgets do not stop a scroll"
+    );
+    assert_eq!(clicked.take(), []);
+    assert_eq!(overlay_clicked.take(), []);
+}
+
+#[test]
+fn hovering_a_button_above_does_not_stop_a_later_scroll() {
+    let mut h = harness();
+    let (list, scrolled) = listener(&mut h, CursorSense::Scroll);
+    let (button, _clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((list, button).stack());
+
+    // The hover arrives in its own frame, as a window delivers it.
+    h.move_to((50, 50));
+    assert_eq!(scrolled.take(), []);
+
+    h.scroll((0, 10));
+    assert_eq!(
+        scrolled.take(),
+        [CursorSense::Scroll],
+        "a hover already resting on the button must not consume the wheel"
+    );
+}
+
+#[test]
+fn only_the_topmost_listener_takes_a_press() {
+    let mut h = harness();
+    let (below, below_clicked) = listener(&mut h, CursorSense::click());
+    let (above, above_clicked) = listener(&mut h, CursorSense::click());
+    h.set_root((below, above).stack());
+
+    h.click((50, 50));
+
+    assert_eq!(above_clicked.take(), [CursorSense::click()]);
+    assert_eq!(below_clicked.take(), [], "one press goes to one widget");
+}
+
+#[test]
+fn a_press_beside_the_button_reaches_the_layer_below() {
+    let mut h = harness();
+    let (list, list_clicked) = listener(&mut h, CursorSense::click());
+    // The row above the list covers it, but only its left half is the button.
+    let (button, button_clicked) = listener(&mut h, CursorSense::click());
+    let row = (button, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((list, row).stack());
+
+    h.click((20, 50));
+    assert_eq!(button_clicked.take(), [CursorSense::click()]);
+    assert_eq!(list_clicked.take(), []);
+
+    h.click((80, 50));
+    assert_eq!(button_clicked.take(), [], "the cursor is not on the button");
+    assert_eq!(
+        list_clicked.take(),
+        [CursorSense::click()],
+        "a press beside the button belongs to what is under it"
+    );
+}
+
+#[test]
+fn leaving_a_widget_still_ends_its_hover() {
+    let mut h = harness();
+    let (widget, hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    h.set_root(widget);
+
+    h.move_to((50, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    h.leave();
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+}
+
+#[test]
+fn leaving_a_widget_does_not_block_the_layer_below() {
+    let mut h = harness();
+    let (below, below_hover) = listener(&mut h, CursorSense::HoverStart);
+    // Only the left half of the layer above is a widget, so the cursor can
+    // leave it without leaving the one underneath.
+    let (above, above_hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let row = (above, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((below, row).stack());
+
+    h.move_to((20, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(below_hover.take(), [], "the layer above is over it");
+
+    h.move_to((80, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverEnd]);
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverStart],
+        "ending a hover above must not stop the hover below"
+    );
+}
+
+#[test]
+fn covering_a_widget_ends_its_hover() {
+    let mut h = harness();
+    let (below, below_hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let (above, above_hover) = listener(&mut h, CursorSense::HoverStart);
+    let row = (above, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root((below, row).stack());
+
+    h.move_to((80, 50));
+    assert_eq!(below_hover.take(), [CursorSense::HoverStart]);
+
+    h.move_to((20, 50));
+    assert_eq!(above_hover.take(), [CursorSense::HoverStart]);
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverEnd],
+        "a widget covered by one that took the input is no longer hovered"
+    );
+
+    h.move_to((80, 50));
+    assert_eq!(
+        below_hover.take(),
+        [CursorSense::HoverStart],
+        "uncovering it hovers it again"
+    );
+}
+
+#[test]
+fn hover_starts_and_ends_once_each() {
+    let mut h = harness();
+    // Only the left half is the widget, so the cursor can leave it without
+    // leaving the window.
+    let (widget, hover) = listener(&mut h, CursorSense::HoverStart | CursorSense::HoverEnd);
+    let row = (widget, blank(&mut h)).span(Dir::RIGHT).add(&mut h.rsc);
+    h.set_root(row);
+
+    h.move_to((20, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverStart]);
+
+    h.move_to((30, 50));
+    assert_eq!(hover.take(), [], "staying inside is not a second start");
+
+    h.move_to((80, 50));
+    assert_eq!(hover.take(), [CursorSense::HoverEnd]);
+
+    h.move_to((90, 50));
+    assert_eq!(hover.take(), [], "an ended hover does not end again");
+
+    h.move_to((20, 50));
+    assert_eq!(
+        hover.take(),
+        [CursorSense::HoverStart],
+        "re-entering starts it"
+    );
+}