WIP: instant-set for zero duration

Fixes #4399

src/runtime/motion/tweened.ts

@@ -93,6 +93,11 @@ export function tweened<T>(value?: T, defaults: Options<T> = {}): Tweened<T> {
 			interpolate = get_interpolator
 		} = assign(assign({}, defaults), opts);
 
+		if (duration === 0) {
+			store.set(target_value);
+			return Promise.resolve();
+		}
+
 		const start = now() + delay;
 		let fn;
 

test/motion/index.js

@@ -19,5 +19,12 @@ describe('motion', () => {
 			size.set(100);
 			assert.equal(get(size), 100);
 		});
+
+		it('sets immediately when duration is 0', () => {
+			const size = tweened(0);
+
+			size.set(100, { duration : 0 });
+			assert.equal(get(size), 100);
+		});
 	});
 });