Reset scroll position after navigation. Fixes #10482

Author: SteveSandersonMS

src/Components/Web.JS/dist/Release/blazor.webassembly.js

@@ -9,6 +9,7 @@ interface BrowserRendererRegistry {
   [browserRendererId: number]: BrowserRenderer;
 }
 const browserRenderers: BrowserRendererRegistry = {};
+let shouldResetScrollAfterNextBatch = false;
 
 export function attachRootComponentToLogicalElement(browserRendererId: number, logicalElement: LogicalElement, componentId: number): void {
 
@@ -67,4 +68,20 @@ export function renderBatch(browserRendererId: number, batch: RenderBatch): void
     const eventHandlerId = batch.disposedEventHandlerIdsEntry(disposedEventHandlerIdsValues, i);
     browserRenderer.disposeEventHandler(eventHandlerId);
   }
+
+  resetScrollIfNeeded();
+}
+
+export function resetScrollAfterNextBatch() {
+  shouldResetScrollAfterNextBatch = true;
+}
+
+function resetScrollIfNeeded() {
+  if (shouldResetScrollAfterNextBatch) {
+    shouldResetScrollAfterNextBatch = false;
+
+    // This assumes the scroller is on the window itself. There isn't a general way to know
+    // if some other element is playing the role of the primary scroll region.
+    window.scrollTo && window.scrollTo(0, 0);
+  }
 }

src/Components/Web.JS/src/Rendering/Renderer.ts

@@ -1,4 +1,5 @@
 import '@dotnet/jsinterop';
+import { resetScrollAfterNextBatch } from '../Rendering/Renderer';
 
 let hasRegisteredNavigationInterception = false;
 let hasRegisteredNavigationEventListeners = false;
@@ -81,6 +82,13 @@ export function navigateTo(uri: string, forceLoad: boolean) {
 }
 
 function performInternalNavigation(absoluteInternalHref: string, interceptedLink: boolean) {
+  // Since this was *not* triggered by a back/forward gesture (that goes through a different
+  // code path starting with a popstate event), we don't want to preserve the current scroll
+  // position, so reset it.
+  // To avoid ugly flickering effects, we don't want to change the scroll position until the
+  // we render the new page. As a best approximation, wait until the next batch.
+  resetScrollAfterNextBatch();
+
   history.pushState(null, /* ignored title */ '', absoluteInternalHref);
   notifyLocationChanged(interceptedLink);
 }

src/Components/Web.JS/src/Services/UriHelper.ts

@@ -418,6 +418,40 @@ public void RoutingToComponentOutsideMainAppDoesNotWork()
             Assert.Equal("Oops, that component wasn't found!", app.FindElement(By.Id("test-info")).Text);
         }
 
+        [Fact]
+        public void ResetsScrollPositionWhenPerformingInternalNavigation_LinkClick()
+        {
+            SetUrlViaPushState("/LongPage1");
+            var app = MountTestComponent<TestRouter>();
+            Browser.Equal("This is a long page you can scroll.", () => app.FindElement(By.Id("test-info")).Text);
+            BrowserScrollY = 500;
+            Browser.True(() => BrowserScrollY > 300); // Exact position doesn't matter
+
+            app.FindElement(By.LinkText("Long page 2")).Click();
+            Browser.Equal("This is another long page you can scroll.", () => app.FindElement(By.Id("test-info")).Text);
+            Browser.Equal(0, () => BrowserScrollY);
+        }
+
+        [Fact]
+        public void ResetsScrollPositionWhenPerformingInternalNavigation_ProgrammaticNavigation()
+        {
+            SetUrlViaPushState("/LongPage1");
+            var app = MountTestComponent<TestRouter>();
+            Browser.Equal("This is a long page you can scroll.", () => app.FindElement(By.Id("test-info")).Text);
+            BrowserScrollY = 500;
+            Browser.True(() => BrowserScrollY > 300); // Exact position doesn't matter
+
+            app.FindElement(By.Id("go-to-longpage2")).Click();
+            Browser.Equal("This is another long page you can scroll.", () => app.FindElement(By.Id("test-info")).Text);
+            Browser.Equal(0, () => BrowserScrollY);
+        }
+
+        private long BrowserScrollY
+        {
+            get => (long)((IJavaScriptExecutor)Browser).ExecuteScript("return window.scrollY");
+            set => ((IJavaScriptExecutor)Browser).ExecuteScript($"window.scrollTo(0, {value})");
+        }
+
         private string SetUrlViaPushState(string relativeUri)
         {
             var pathBaseWithoutHash = ServerPathBase.Split('#')[0];

src/Components/test/E2ETest/Tests/RoutingTest.cs

@@ -18,6 +18,8 @@
     <li><NavLink href="/subdir/Other#blah">Other with hash</NavLink></li>
     <li><NavLink href="/subdir/WithParameters/Name/Abc">With parameters</NavLink></li>
     <li><NavLink href="/subdir/WithParameters/Name/Abc/LastName/McDef">With more parameters</NavLink></li>
+    <li><NavLink href="/subdir/LongPage1">Long page 1</NavLink></li>
+    <li><NavLink href="/subdir/LongPage2">Long page 2</NavLink></li>
 </ul>
 
 <button id="do-navigation" @onclick=@(x => uriHelper.NavigateTo("Other"))>

src/Components/test/testassets/BasicTestApp/RouterTest/Links.razor

@@ -0,0 +1,13 @@
+@page "/LongPage1"
+@inject IUriHelper UriHelper
+<div id="test-info">This is a long page you can scroll.</div>
+
+<div style="border: 2px dashed red; margin: 1rem; padding: 1rem; height: 1500px;">
+    Scroll past me to find the links
+</div>
+
+<button id="go-to-longpage2" @onclick="@(() => UriHelper.NavigateTo("LongPage2"))">
+    Navigate programmatically to long page 2
+</button>
+
+<Links />

src/Components/test/testassets/BasicTestApp/RouterTest/LongPage1.razor

@@ -0,0 +1,8 @@
+@page "/LongPage2"
+<div id="test-info">This is another long page you can scroll.</div>
+
+<div style="border: 2px dashed blue; margin: 1rem; padding: 1rem; height: 1500px;">
+    Scroll past me to find the links
+</div>
+
+<Links />