Add Transition::jump_to for transitions (#58)

crates/gpui/src/transition.rs

@@ -208,6 +208,37 @@ impl<T: Lerp + Clone + PartialEq + 'static> Transition<T> {
         was_updated
     }
 
+    /// Instantly set the transition to the given target value without animation.
+    ///
+    /// Sets both the start and end goals to `target` so that subsequent evaluations
+    /// return `target` immediately. This is useful for transitions that require a
+    /// different start value on each update.
+    pub fn jump_to(&self, target: T, cx: &mut App) {
+        self.state.update(cx, |state, _cx| {
+            state.start_goal = target.clone();
+            state.end_goal = target;
+            state.goal_last_updated_at = Some(Instant::now());
+        });
+        self.cached_value.borrow_mut().take();
+    }
+
+    /// Scale the transition's start and end goals by the given ratio.
+    ///
+    /// This preserves the relative progress of an in-flight animation when the
+    /// coordinate space changes (e.g. on window resize).  Both the start and
+    /// end goal are multiplied by `ratio` so the interpolated value remains
+    /// proportionally correct.
+    pub fn scale_by(&self, ratio: f32, cx: &mut App)
+    where
+        T: std::ops::Mul<f32, Output = T>,
+    {
+        self.state.update(cx, |state, _cx| {
+            state.start_goal = state.start_goal.clone() * ratio;
+            state.end_goal = state.end_goal.clone() * ratio;
+        });
+        self.cached_value.borrow_mut().take();
+    }
+
     /// Returns the entity ID associated with this transition's state.
     ///
     /// This can be useful for tracking or comparing transitions.