Fix session list selection when adding new session in side-by-side view (#290233)

* Initial plan

* Fix session list selection when adding new session in side-by-side view

- Clear list selection when creating a new untitled session
- Reveal session in list after first message is submitted
- Change reveal() return type from void to boolean
- Add onDidChangeSessions listener to auto-select session when it appears

Co-authored-by: bpasero <900690+bpasero@users.noreply.github.com>

* Add comment explaining why reveal() return value is ignored in onDidChangeSessions

Co-authored-by: bpasero <900690+bpasero@users.noreply.github.com>

* .

* .

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: bpasero <900690+bpasero@users.noreply.github.com>
Co-authored-by: Benjamin Pasero <benjamin.pasero@microsoft.com>

src/vs/workbench/contrib/chat/browser/agentSessions/agentSessions.ts

@@ -117,7 +117,11 @@ export interface IAgentSessionsControl {
 
 	refresh(): void;
 	openFind(): void;
-	reveal(sessionResource: URI): void;
+
+	reveal(sessionResource: URI): boolean;
+
+	clearFocus(): void;
+	hasFocusOrSelection(): boolean;
 }
 
 export const agentSessionReadIndicatorForeground = registerColor(

src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionsControl.ts

@@ -371,6 +371,10 @@ export class AgentSessionsControl extends Disposable implements IAgentSessionsCo
 		this.sessionsList?.setSelection([]);
 	}
 
+	hasFocusOrSelection(): boolean {
+		return (this.sessionsList?.getFocus().length ?? 0) > 0 || (this.sessionsList?.getSelection().length ?? 0) > 0;
+	}
+
 	scrollToTop(): void {
 		if (this.sessionsList) {
 			this.sessionsList.scrollTop = 0;
@@ -383,14 +387,14 @@ export class AgentSessionsControl extends Disposable implements IAgentSessionsCo
 		return focused.filter(e => isAgentSession(e));
 	}
 
-	reveal(sessionResource: URI): void {
+	reveal(sessionResource: URI): boolean {
 		if (!this.sessionsList) {
-			return;
+			return false;
 		}
 
 		const session = this.agentSessionsService.model.getSession(sessionResource);
 		if (!session || !this.sessionsList.hasNode(session)) {
-			return;
+			return false;
 		}
 
 		if (this.sessionsList.getRelativeTop(session) === null) {
@@ -399,5 +403,7 @@ export class AgentSessionsControl extends Disposable implements IAgentSessionsCo
 
 		this.sessionsList.setFocus([session]);
 		this.sessionsList.setSelection([session]);
+
+		return true;
 	}
 }

src/vs/workbench/contrib/chat/browser/widgetHosts/viewPane/chatViewPane.ts

@@ -594,6 +594,28 @@ export class ChatViewPane extends ViewPane implements IViewWelcomeDelegate {
 				return; // only reveal in side-by-side mode
 			}
 
+			const sessionResource = chatWidget.viewModel?.sessionResource;
+			if (sessionResource) {
+				const revealed = sessionsControl.reveal(sessionResource);
+				if (!revealed) {
+					// Session doesn't exist in the list yet (e.g., new untitled session),
+					// clear the selection so the list doesn't show stale selection
+					sessionsControl.clearFocus();
+				}
+			}
+		}));
+
+		// When sessions change (e.g., after first message in a new session)
+		// reveal it unless the user is interacting with the list already
+		this._register(this.agentSessionsService.model.onDidChangeSessions(() => {
+			if (this.sessionsViewerOrientation === AgentSessionsViewerOrientation.Stacked) {
+				return; // only reveal in side-by-side mode
+			}
+
+			if (sessionsControl.hasFocusOrSelection()) {
+				return; // do not reveal if user is interacting with sessions control
+			}
+
 			const sessionResource = chatWidget.viewModel?.sessionResource;
 			if (sessionResource) {
 				sessionsControl.reveal(sessionResource);