Run the overscroll animation all the way to the destination

This fixes a bug where a layer could get "stuck" in overscroll because it gets
within 0.1 of the edge and then stops.

src/frame.rs

@@ -1301,10 +1301,7 @@ impl Frame {
         let mut layers_bouncing_back = HashSet::with_hasher(Default::default());
         for (scroll_layer_id, layer) in &self.layers {
             if layer.scrolling.started_bouncing_back {
-                let overscroll_amount = layer.overscroll_amount();
-                if overscroll_amount.width.abs() >= 0.1 || overscroll_amount.height.abs() >= 0.1 {
-                    layers_bouncing_back.insert(*scroll_layer_id);
-                }
+                layers_bouncing_back.insert(*scroll_layer_id);
             }
         }
         layers_bouncing_back

src/layer.rs

@@ -114,8 +114,11 @@ impl Layer {
     }
 
     pub fn tick_scrolling_bounce_animation(&mut self) {
-        self.scrolling.spring.animate();
+        let finished = self.scrolling.spring.animate();
         self.scrolling.offset = self.scrolling.spring.current();
+        if finished {
+            self.scrolling.started_bouncing_back = false;
+        }
     }
 }
 

src/spring.rs

@@ -50,7 +50,8 @@ impl Spring {
         self.cur
     }
 
-    pub fn animate(&mut self) {
+    /// Run one tick of the spring animation. Return true if the animation is complete.
+    pub fn animate(&mut self) -> bool {
         if !is_resting(self.cur.x, self.prev.x, self.dest.x) ||
                 !is_resting(self.cur.y, self.prev.y, self.dest.y) {
             let next = Point2D::new(next(self.cur.x,
@@ -64,10 +65,12 @@ impl Spring {
                                          self.stiffness,
                                          self.damping));
             let (cur, dest) = (self.cur, self.dest);
-            self.coords(next, cur, dest)
+            self.coords(next, cur, dest);
+            false
         } else {
             let dest = self.dest;
-            self.coords(dest, dest, dest)
+            self.coords(dest, dest, dest);
+            true
         }
     }
 }