Avoid UI flickering during transition from prerendered to live content

src/Components/Browser.JS/src/Rendering/BrowserRenderer.ts

@@ -7,6 +7,7 @@ const selectValuePropname = '_blazorSelectValue';
 const sharedTemplateElemForParsing = document.createElement('template');
 const sharedSvgElemForParsing = document.createElementNS('http://www.w3.org/2000/svg', 'g');
 const preventDefaultEvents: { [eventType: string]: boolean } = { submit: true };
+const rootComponentsPendingFirstRender: { [componentId: number]: Element } = {};
 
 export class BrowserRenderer {
   private eventDelegator: EventDelegator;
@@ -19,7 +20,9 @@ export class BrowserRenderer {
   }
 
   public attachRootComponentToElement(componentId: number, element: Element) {
-    this.attachComponentToElement(componentId, toLogicalElement(element));
+    // 'allowExistingContents' to keep any prerendered content until we do the first client-side render
+    this.attachComponentToElement(componentId, toLogicalElement(element, /* allowExistingContents */ true));
+    rootComponentsPendingFirstRender[componentId] = element;
   }
 
   public updateComponent(batch: RenderBatch, componentId: number, edits: ArraySegment<RenderTreeEdit>, referenceFrames: ArrayValues<RenderTreeFrame>) {
@@ -28,6 +31,13 @@ export class BrowserRenderer {
       throw new Error(`No element is currently associated with component ${componentId}`);
     }
 
+    // On the first render for each root component, clear any existing content (e.g., prerendered)
+    const rootElementToClear = rootComponentsPendingFirstRender[componentId];
+    if (rootElementToClear) {
+      delete rootComponentsPendingFirstRender[componentId];
+      clearElement(rootElementToClear);
+    }
+
     this.applyEdits(batch, element, 0, edits, referenceFrames);
   }
 
@@ -368,3 +378,10 @@ function raiseEvent(event: Event, browserRendererId: number, eventHandlerId: num
     eventDescriptor,
     JSON.stringify(eventArgs.data));
 }
+
+function clearElement(element: Element) {
+  let childNode: Node | null;
+  while (childNode = element.firstChild) {
+    element.removeChild(childNode);
+  }
+}

src/Components/Browser.JS/src/Rendering/LogicalElements.ts

@@ -28,9 +28,12 @@
 const logicalChildrenPropname = createSymbolOrFallback('_blazorLogicalChildren');
 const logicalParentPropname = createSymbolOrFallback('_blazorLogicalParent');
 
-export function toLogicalElement(element: Element) {
+export function toLogicalElement(element: Element, allowExistingContents?: boolean) {
-  if (element.childNodes.length > 0) {
+  // Normally it's good to assert that the element has started empty, because that's the usual
-    throw new Error('New logical elements must start empty');
+  // situation and we probably have a bug if it's not. But for the element that contain prerendered
+  // root components, we want to let them keep their content until we replace it.
+  if (element.childNodes.length > 0 && !allowExistingContents) {
+    throw new Error('New logical elements must start empty, or allowExistingContents must be true');
   }
 
   element[logicalChildrenPropname] = [];

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

@@ -16,7 +16,6 @@ export function attachRootComponentToElement(browserRendererId: number, elementS
   if (!browserRenderer) {
     browserRenderer = browserRenderers[browserRendererId] = new BrowserRenderer(browserRendererId);
   }
-  clearElement(element);
   browserRenderer.attachRootComponentToElement(componentId, element);
 }
 
@@ -57,10 +56,3 @@ export function renderBatch(browserRendererId: number, batch: RenderBatch) {
     browserRenderer.disposeEventHandler(eventHandlerId);
   }
 }
-
-function clearElement(element: Element) {
-  let childNode: Node | null;
-  while (childNode = element.firstChild) {
-    element.removeChild(childNode);
-  }
-}