Smoother animations (#4787)

This makes animations slightly smoother, especially in reactive mode

crates/egui/src/animation_manager.rs

@@ -87,8 +87,8 @@ impl AnimationManager {
             Some(anim) => {
                 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;
+                // so we extrapolate forwards by half a frame:
+                let time_since_toggle = time_since_toggle + input.predicted_dt / 2.0;
                 let current_value = remap_clamp(
                     time_since_toggle,
                     0.0..=animation_time,

crates/egui/src/containers/area.rs

@@ -545,9 +545,10 @@ impl Prepared {
 
         if self.fade_in {
             if let Some(last_became_visible_at) = self.state.last_became_visible_at {
-                let age = ctx.input(|i| (i.time - last_became_visible_at) as f32 + i.predicted_dt);
+                let age =
+                    ctx.input(|i| (i.time - last_became_visible_at) as f32 + i.predicted_dt / 2.0);
                 let opacity = crate::remap_clamp(age, 0.0..=ctx.style().animation_time, 0.0..=1.0);
-                let opacity = emath::easing::cubic_out(opacity); // slow fade-out = quick fade-in
+                let opacity = emath::easing::quadratic_out(opacity); // slow fade-out = quick fade-in
                 ui.multiply_opacity(opacity);
                 if opacity < 1.0 {
                     ctx.request_repaint();

crates/egui/src/context.rs

@@ -146,7 +146,7 @@ impl ContextImpl {
 
     fn request_repaint_after(
         &mut self,
-        delay: Duration,
+        mut delay: Duration,
         viewport_id: ViewportId,
         cause: RepaintCause,
     ) {
@@ -163,6 +163,11 @@ impl ContextImpl {
             // Hovering a tooltip is a good example of a case where we want to repaint after a delay.
         }
 
+        if let Ok(predicted_frame_time) = Duration::try_from_secs_f32(viewport.input.predicted_dt) {
+            // Make it less likely we over-shoot the target:
+            delay = delay.saturating_sub(predicted_frame_time);
+        }
+
         viewport.repaint.causes.push(cause);
 
         // We save some CPU time by only calling the callback if we need to.