Properly reverse bool animation if value changes before it's finished (#3577)

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Closes <https://github.com/emilk/egui/issues/3576>.

crates/egui/src/animation_manager.rs

@@ -1,4 +1,7 @@
-use crate::{emath::remap_clamp, Id, IdMap, InputState};
+use crate::{
+    emath::{remap_clamp, NumExt as _},
+    Id, IdMap, InputState,
+};
 
 #[derive(Clone, Default)]
 pub(crate) struct AnimationManager {
@@ -8,10 +11,8 @@ pub(crate) struct AnimationManager {
 
 #[derive(Clone, Debug)]
 struct BoolAnim {
-    value: bool,
-
-    /// when did `value` last toggle?
-    toggle_time: f64,
+    last_value: f32,
+    last_tick: f64,
 }
 
 #[derive(Clone, Debug)]
@@ -33,38 +34,33 @@ impl AnimationManager {
         id: Id,
         value: bool,
     ) -> f32 {
+        let (start, end) = if value { (0.0, 1.0) } else { (1.0, 0.0) };
         match self.bools.get_mut(&id) {
             None => {
                 self.bools.insert(
                     id,
                     BoolAnim {
-                        value,
-                        toggle_time: -f64::INFINITY, // long time ago
+                        last_value: end,
+                        last_tick: input.time - input.stable_dt as f64,
                     },
                 );
-                if value {
-                    1.0
-                } else {
-                    0.0
-                }
+                end
             }
             Some(anim) => {
-                if anim.value != value {
-                    anim.value = value;
-                    anim.toggle_time = input.time;
-                }
-
-                let time_since_toggle = (input.time - anim.toggle_time) as f32;
-
-                // On the frame we toggle we don't want to return the old value,
-                // so we extrapolate forwards:
-                let time_since_toggle = time_since_toggle + input.predicted_dt;
-
-                if value {
-                    remap_clamp(time_since_toggle, 0.0..=animation_time, 0.0..=1.0)
+                let BoolAnim {
+                    last_value,
+                    last_tick,
+                } = anim;
+                let current_time = input.time;
+                let elapsed = ((current_time - *last_tick) as f32).at_most(input.stable_dt);
+                let new_value = *last_value + (end - start) * elapsed / animation_time;
+                *last_value = if new_value.is_finite() {
+                    new_value.clamp(0.0, 1.0)
                 } else {
-                    remap_clamp(time_since_toggle, 0.0..=animation_time, 1.0..=0.0)
-                }
+                    end
+                };
+                *last_tick = current_time;
+                *last_value
             }
         }
     }