bread-theme: clamp spring_to's interpolated value to [from,to]

spring_ease's backOut overshoot (peaks ~1.065) drove a shrinking
animation's interpolated value below its target, which could go
negative for a size request and trip GTK's height >= -1 assertion.
Previously only patched at breadbar's call sites; clamp inside
spring_to itself so the safety is automatic for every caller.

bread-theme/src/anim.rs

@@ -29,6 +29,28 @@ fn spring_ease(t: f64) -> f64 {
     1.0 + t1 * t1 * ((c + 1.0) * t1 + c)
 }
 
+/// One frame's worth of `spring_to`'s interpolation math — split out from
+/// the tick callback below purely so it has a name a unit test can call
+/// directly instead of re-deriving the same arithmetic.
+///
+/// `spring_ease` is a backOut curve that legitimately overshoots past 1.0
+/// (peaks ~1.065) partway through the run — fine for a growing animation
+/// (`to` > `from`), but on a *shrink* (`from` > `to`, e.g. a drawer
+/// collapsing to 0) that overshoot drives the raw interpolated value BELOW
+/// `to`, which can go negative for a size request and trip GTK's
+/// `height >= -1` assertion. Clamping here, once, protects every caller
+/// automatically instead of relying on each call site to remember
+/// `h.max(0)` — this already bit breadbar once (see
+/// `breadbar::main::animate_drawer_height`'s own clamp, now
+/// redundant-but-harmless defense in depth on top of this).
+fn frame_value(from: i32, to: i32, t: f64) -> i32 {
+    let eased = spring_ease(t);
+    let value = from as f64 + (to - from) as f64 * eased;
+    let lo = from.min(to) as f64;
+    let hi = from.max(to) as f64;
+    value.clamp(lo, hi).round() as i32
+}
+
 /// Interpolates from `from` to `to` over `duration_ms`, calling `on_frame`
 /// with each intermediate value (and, on the final tick, the exact `to` —
 /// never an off-by-rounding near-miss) via `widget`'s frame clock.
@@ -60,9 +82,61 @@ pub fn spring_to(
             on_frame(to);
             return ControlFlow::Break;
         }
-        let t = spring_ease(elapsed / duration_ms);
-        let value = from as f64 + (to - from) as f64 * t;
-        on_frame(value.round() as i32);
+        on_frame(frame_value(from, to, elapsed / duration_ms));
         ControlFlow::Continue
     })
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Samples [`frame_value`] — the exact function `spring_to`'s tick
+    /// callback calls every frame — across the full `t` timeline at fine
+    /// granularity, bypassing the real frame clock (which needs a running
+    /// main loop the test environment doesn't have).
+    fn sample_all_frames(from: i32, to: i32) -> Vec<i32> {
+        let steps = 2000;
+        (0..=steps)
+            .map(|i| frame_value(from, to, i as f64 / steps as f64))
+            .collect()
+    }
+
+    #[test]
+    fn spring_ease_overshoots_past_one_for_a_backout_curve() {
+        // Sanity check on the premise: without a clamp, a shrink would
+        // legitimately go negative around this point in the curve.
+        assert!(spring_ease(0.8) > 1.0, "expected overshoot past 1.0");
+    }
+
+    #[test]
+    fn shrink_never_emits_a_value_outside_the_from_to_range() {
+        // from > to (a drawer collapsing to 0) is exactly the case the
+        // unclamped overshoot could drive negative.
+        for value in sample_all_frames(480, 0) {
+            assert!(
+                (0..=480).contains(&value),
+                "shrink emitted {value}, outside [0, 480]"
+            );
+        }
+    }
+
+    #[test]
+    fn grow_never_emits_a_value_outside_the_from_to_range() {
+        for value in sample_all_frames(0, 480) {
+            assert!(
+                (0..=480).contains(&value),
+                "grow emitted {value}, outside [0, 480]"
+            );
+        }
+    }
+
+    #[test]
+    fn zero_span_never_clamps_outside_the_single_point() {
+        // from == to: lo == hi == that point for every t, including past
+        // the overshoot peak.
+        for value in sample_all_frames(120, 120) {
+            assert_eq!(value, 120);
+        }
+    }
+}