only replace components for changed segments

test/app/routes/[x]/[y]/[z].html

@@ -0,0 +1,20 @@
+<span>z: {segment} {count}</span>
+<a href="foo/bar/qux"></a>
+
+<script>
+	import counts from '../_counts.js';
+
+	export default {
+		preload() {
+			return {
+				count: counts.z += 1
+			};
+		},
+
+		oncreate() {
+			this.set({
+				segment: this.get().params.z
+			});
+		}
+	};
+</script>

test/app/routes/[x]/[y]/index.html

@@ -0,0 +1,20 @@
+<span>y: {segment} {count}</span>
+<svelte:component this={child.component} {...child.props}/>
+
+<script>
+	import counts from '../_counts.js';
+
+	export default {
+		preload() {
+			return {
+				count: counts.y += 1
+			};
+		},
+
+		oncreate() {
+			this.set({
+				segment: this.get().params.y
+			});
+		}
+	};
+</script>

test/app/routes/[x]/_counts.js

@@ -0,0 +1,5 @@
+export default {
+	x: process.browser ? 1 : 0,
+	y: process.browser ? 1 : 0,
+	z: process.browser ? 1 : 0
+};

test/app/routes/[x]/index.html

@@ -0,0 +1,20 @@
+<span>x: {segment} {count}</span>
+<svelte:component this={child.component} {...child.props}/>
+
+<script>
+	import counts from './_counts.js';
+
+	export default {
+		preload() {
+			return {
+				count: counts.x += 1
+			};
+		},
+
+		oncreate() {
+			this.set({
+				segment: this.get().params.x
+			});
+		}
+	};
+</script>

test/common/test.js

@@ -632,6 +632,33 @@ function run({ mode, basepath = '' }) {
 						assert.equal(html.indexOf('%sapper'), -1);
 					});
 			});
+
+			it('only recreates components when necessary', () => {
+				return nightmare
+					.goto(`${base}/foo/bar/baz`)
+					.init()
+					.evaluate(() => document.querySelector('#sapper').textContent)
+					.then(text => {
+						assert.deepEqual(text.split('\n').filter(Boolean), [
+							'x: foo 1',
+							'y: bar 1',
+							'z: baz 1'
+						]);
+
+						return nightmare.click(`a`)
+							.then(() => wait(100))
+							.then(() => {
+								return nightmare.evaluate(() => document.querySelector('#sapper').textContent);
+							});
+					})
+					.then(text => {
+						assert.deepEqual(text.split('\n').filter(Boolean), [
+							'x: foo 1',
+							'y: bar 1',
+							'z: qux 2'
+						]);
+					});
+			});
 		});
 
 		describe('headers', () => {