sessions: gate titlebar new-session button behind experiment + add telemetry

[sandy081]

Follow-up to #322614 addressing reviewer feedback to ship the titlebar New Session button as an A/B experiment and measure click-through.

What changed

• Experiment gating — the titlebar New Session button now only shows for the agentSessionsTitleBarNewSession ExP treatment group. NewSessionActionViewItemContribution resolves the treatment (and re-resolves on onDidRefetchAssignments) and reflects it into the new SessionsTitleBarNewSessionEnabledContext context key, which the Menus.TitleBarLeftLayout menu when clause checks. The control group keeps the prior behaviour (no titlebar button), so we can see how the affordance moves new-session metrics before a wider rollout.
• Per-surface telemetry — the vscodeAgents.interaction event now records the originating surface (titleBar vs sidebar) for the new-session action, so we can count how often the button is clicked specifically from the titlebar.

How it addresses the feedback

like the idea and I'd love that we build our exp. Could we please do this via an experiment and see how metrics move?

Done via the agentSessionsTitleBarNewSession treatment — the button is off by default and only enabled for the experiment's treatment group.

Also it would be nice to know how many times user click this button from title bar

Done — the interaction telemetry now distinguishes titleBar vs sidebar as the source.

Validation

• npm run typecheck-client ✅
• npm run valid-layers-check ✅
• eslint on changed files ✅
• hygiene ✅

[Copilot]

Pull request overview

This PR follows up on Agents window UX feedback by gating the titlebar New Session affordance behind an ExP treatment and by extending interaction telemetry to attribute new session clicks to a specific UI surface (titlebar vs sidebar).

Changes:

• Gate the titlebar New Session button behind the agentSessionsTitleBarNewSession experiment by reflecting treatment state into a new SessionsTitleBarNewSessionEnabledContext context key.
• Move the sidebar header New Session affordance to a shared NewSessionActionViewItem so the sidebar header and titlebar render the same compact “New” pill.
• Extend vscodeAgents.interaction telemetry source values to include titleBar and sidebar.

Show a summary per file

File	Description
src/vs/sessions/LAYOUT.md	Updates layout docs to mention the (experiment-gated) titlebar New Session affordance.
src/vs/sessions/contrib/sessions/browser/views/sessionsView.ts	Removes the hand-rendered header New Session button and relies on a menu-driven toolbar.
src/vs/sessions/contrib/sessions/browser/sessions.contribution.ts	Registers the new NewSessionActionViewItemContribution.
src/vs/sessions/contrib/sessions/browser/newSessionActionViewItem.ts	Adds shared action view item rendering + experiment-driven context key update.
src/vs/sessions/contrib/sessions/browser/media/sessionsViewPane.css	Removes inline styling for the old header button (now shared).
src/vs/sessions/contrib/sessions/browser/media/newSessionActionViewItem.css	Introduces shared styling for the compact “New” pill across surfaces.
src/vs/sessions/contrib/chat/browser/chat.contribution.ts	Contributes the new-session action to sidebar header + titlebar menus with when gating.
src/vs/sessions/common/sessionsTelemetry.ts	Expands interaction source values to include titleBar and sidebar.
src/vs/sessions/common/contextkeys.ts	Adds SessionsTitleBarNewSessionEnabledContext context key for the experiment gate.

Copilot's findings

Comments suppressed due to low confidence (2)

src/vs/sessions/contrib/sessions/browser/newSessionActionViewItem.ts:66

• This view item renders a focusable Button () inside an ActionBar item. ActionBar manages focus/tab stops via focus()/setFocusable() on the view item (see ActionViewItem), so having an additional tabbable element can create confusing keyboard/tab order and make activation paths inconsistent (e.g. arrow-key toolbar activation goes through ActionBar’s trigger path, bypassing the button’s handler/telemetry and its aria-label). Consider delegating focusability to the inner button (override focus/blur/setFocusable like ActionViewItem does) or render directly on the container element (similar to OpenInVSCodeTitleBarWidget) so there is exactly one focus target.

		const newSessionButton = this._register(new Button(this.element, {
			...defaultButtonStyles,
			buttonSecondaryBackground: asCssVariable(agentsNewSessionButtonBackground),
			buttonSecondaryForeground: asCssVariable(agentsNewSessionButtonForeground),

src/vs/sessions/common/sessionsTelemetry.ts:36

• The JSDoc still says this logs a "titlebar" interaction, but SessionsInteractionSource now includes sidebar (and other surfaces). Please update the doc comment so telemetry consumers don’t misinterpret the event.

/**
 * Log a titlebar button interaction in the Agents window.
 */
export function logSessionsInteraction(telemetryService: ITelemetryService, button: SessionsInteractionButton, source?: SessionsInteractionSource): void {
	telemetryService.publicLog2<SessionsInteractionEvent, SessionsInteractionClassification>('vscodeAgents.interaction', source ? { button, source } : { button });

• Files reviewed: 5/5 changed files
• Comments generated: 0

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@benibenj

Matched files:

• src/vs/sessions/browser/parts/media/titlebarpart.css