Coordinate chat scroll restoration

src/client/src/components/ChatView.ts

@@ -10,28 +10,21 @@ export class ChatView extends LitElement {
   @property() sessionId = "";
   @query(".chat") private chat?: HTMLDivElement;
   @state() private pinnedToBottom = true;
-  private restoreAfterUpdate = true;
   private suppressScrollSave = false;
+  private saveScrollTimer?: number;
 
-  protected willUpdate(changed: Map<string, unknown>): void {
-    if (changed.has("sessionId")) {
-      const previousSessionId = changed.get("sessionId");
-      if (typeof previousSessionId === "string" && previousSessionId) this.saveScrollPosition(previousSessionId);
-      this.suppressScrollSave = true;
-      this.pinnedToBottom = true;
-      this.restoreAfterUpdate = true;
-      return;
+  disconnectedCallback(): void {
+    window.clearTimeout(this.saveScrollTimer);
+    super.disconnectedCallback();
   }
+
+  protected willUpdate(): void {
     this.pinnedToBottom = this.isNearBottom();
   }
 
-  protected updated(): void {
-    if (this.restoreAfterUpdate) {
-      this.restoreAfterUpdate = false;
-      this.restoreScrollPosition();
-    } else if (this.pinnedToBottom) {
-      this.scrollToBottom();
-    }
+  protected updated(changed: Map<string, unknown>): void {
+    if (changed.has("sessionId")) return;
+    if (changed.has("messages") && this.pinnedToBottom) this.scrollToBottom();
   }
 
   render() {
@@ -62,7 +55,7 @@ export class ChatView extends LitElement {
 
   private onScroll() {
     this.pinnedToBottom = this.isNearBottom();
-    if (!this.suppressScrollSave) this.saveScrollPosition();
+    if (!this.suppressScrollSave) this.scheduleScrollPositionSave();
   }
 
   private isNearBottom(): boolean {
@@ -81,7 +74,7 @@ export class ChatView extends LitElement {
     });
   }
 
-  private restoreScrollPosition() {
+  restoreScrollPosition() {
     requestAnimationFrame(() => {
       const chat = this.chat;
       const stored = this.readStoredScrollPosition();
@@ -107,7 +100,7 @@ export class ChatView extends LitElement {
     });
   }
 
-  private saveScrollPosition(sessionId = this.sessionId) {
+  saveScrollPosition(sessionId = this.sessionId) {
     const chat = this.chat;
     if (!chat || !sessionId) return;
     try {
@@ -131,6 +124,11 @@ export class ChatView extends LitElement {
     }
   }
 
+  private scheduleScrollPositionSave() {
+    window.clearTimeout(this.saveScrollTimer);
+    this.saveScrollTimer = window.setTimeout(() => this.saveScrollPosition(), 180);
+  }
+
   private readStoredScrollPosition(): { index: number; offset: number } | undefined {
     if (!this.sessionId) return undefined;
     try {

src/client/src/components/PiWebApp.ts

@@ -1,5 +1,5 @@
 import { LitElement, html } from "lit";
-import { customElement, state } from "lit/decorators.js";
+import { customElement, query, state } from "lit/decorators.js";
 import type { Project, SessionInfo, Workspace } from "../api";
 import { initialAppState, type AppState } from "../appState";
 import { ProjectController } from "../controllers/projectController";
@@ -10,6 +10,7 @@ import "./ProjectList";
 import "./WorkspaceList";
 import "./SessionList";
 import "./ChatView";
+import type { ChatView } from "./ChatView";
 import "./PromptEditor";
 import "./StatusBar";
 import "./CommandPicker";
@@ -18,6 +19,7 @@ import { appStyles } from "./shared";
 @customElement("pi-web-poc")
 export class PiWebApp extends LitElement {
   @state() private state: AppState = initialAppState();
+  @query("chat-view") private chatView?: ChatView;
 
   private readonly sessions = new SessionController(
     () => this.state,
@@ -35,7 +37,7 @@ export class PiWebApp extends LitElement {
     (patch) => this.setState(patch),
     this.workspaces,
   );
-  private readonly onPopState = () => void this.restoreRoute(false);
+  private readonly onPopState = () => void this.withChatScrollTransition(() => this.restoreRoute(false));
 
   connectedCallback(): void {
     super.connectedCallback();
@@ -55,7 +57,7 @@ export class PiWebApp extends LitElement {
 
   private async loadProjectsAndRestoreRoute() {
     await this.projects.loadProjects();
-    await this.restoreRoute(false);
+    await this.withChatScrollTransition(() => this.restoreRoute(false));
   }
 
   private async restoreRoute(updateUrl: boolean) {
@@ -66,6 +68,15 @@ export class PiWebApp extends LitElement {
     await this.workspaces.selectProject(project, { workspaceId: route.workspaceId, sessionId: route.sessionId, updateUrl });
   }
 
+  private async withChatScrollTransition(action: () => Promise<void>) {
+    this.chatView?.saveScrollPosition();
+    await action();
+    await this.updateComplete;
+    await this.chatView?.updateComplete;
+    await nextFrame();
+    this.chatView?.restoreScrollPosition();
+  }
+
   private updateUrl() {
     writeRoute({
       projectId: this.state.selectedProject?.id,
@@ -83,9 +94,9 @@ export class PiWebApp extends LitElement {
             <strong>Pi Web POC</strong>
             <button @click=${() => this.projects.addProject()}>+ Project</button>
           </header>
-          <project-list .projects=${state.projects} .selected=${state.selectedProject} .onSelect=${(project: Project) => this.workspaces.selectProject(project)}></project-list>
-          <workspace-list .workspaces=${state.workspaces} .selected=${state.selectedWorkspace} .onSelect=${(workspace: Workspace) => this.workspaces.selectWorkspace(workspace)}></workspace-list>
-          <session-list .sessions=${state.sessions} .selected=${state.selectedSession} .canStart=${!!state.selectedWorkspace} .onStart=${() => this.sessions.startSession()} .onSelect=${(session: SessionInfo) => this.sessions.selectSession(session)}></session-list>
+          <project-list .projects=${state.projects} .selected=${state.selectedProject} .onSelect=${(project: Project) => this.withChatScrollTransition(() => this.workspaces.selectProject(project))}></project-list>
+          <workspace-list .workspaces=${state.workspaces} .selected=${state.selectedWorkspace} .onSelect=${(workspace: Workspace) => this.withChatScrollTransition(() => this.workspaces.selectWorkspace(workspace))}></workspace-list>
+          <session-list .sessions=${state.sessions} .selected=${state.selectedSession} .canStart=${!!state.selectedWorkspace} .onStart=${() => this.withChatScrollTransition(() => this.sessions.startSession())} .onSelect=${(session: SessionInfo) => this.withChatScrollTransition(() => this.sessions.selectSession(session))}></session-list>
         </aside>
         <main>
           ${state.error ? html`<div class="error">${state.error}</div>` : null}
@@ -102,3 +113,7 @@ export class PiWebApp extends LitElement {
 
   static styles = appStyles;
 }
+
+function nextFrame(): Promise<void> {
+  return new Promise((resolve) => requestAnimationFrame(() => resolve()));
+}