When working on the Agent Sessions workbench layout, always follow these guidelines:

1. Read the Specification First

The authoritative specification for the Agent Sessions layout lives at:

src/vs/sessions/LAYOUT.md

Before making any changes to the layout code, read and understand the current spec. It defines:

• The fixed layout structure (grid tree, part positions, default sizes)

• Which parts are included/excluded and their visibility defaults

• Titlebar configuration and custom menu IDs

• Editor modal overlay behavior and sizing

• Part visibility API and events

• Agent session part classes and storage keys

• Workbench contributions and lifecycle

• CSS classes and file structure

2. Keep the Spec in Sync

If you modify the layout implementation, you must update LAYOUT.md to reflect those changes. The spec should always match the code. This includes:

• Adding/removing parts or changing their positions

• Changing default visibility or sizing

• Adding new actions, menus, or contributions

• Modifying the grid structure

• Changing titlebar configuration

• Adding new CSS classes or file structure changes

Update the Revision History table at the bottom of LAYOUT.md with a dated entry describing what changed.

3. Implementation Principles

When proposing or implementing changes, follow these rules from the spec:

• Maintain fixed positions — Do not add settings-based position customization

• Panel must span the right section width — The grid structure places the panel below Chat Bar and Auxiliary Bar only

• Sidebar spans full height — Sidebar is in the main content branch, spanning from top to bottom

• New parts go in the right section — Any new parts should be added to the horizontal branch alongside Chat Bar and Auxiliary Bar

• Preserve no-op methods — Unsupported features (zen mode, centered layout, etc.) should remain as no-ops, not throw errors

• Handle pane composite lifecycle — When hiding/showing parts, manage the associated pane composites

• Use agent session parts — New part functionality goes in the agent session part classes (SidebarPart , AuxiliaryBarPart , PanelPart , ChatBarPart , ProjectBarPart ), not the standard workbench parts

• Use separate storage keys — Agent session parts use their own storage keys (prefixed with workbench.agentsession. or workbench.chatbar. ) to avoid conflicts with regular workbench state

• Use agent session menu IDs — Actions should use Menus.* menu IDs (from sessions/browser/menus.ts ), not shared MenuId.* constants

4. Key Files

File Purpose

sessions/LAYOUT.md

Authoritative layout specification

sessions/browser/workbench.ts

Main layout implementation (Workbench class)

sessions/browser/menus.ts

Agent sessions menu IDs (Menus export)

sessions/browser/layoutActions.ts

Layout actions (toggle sidebar, panel, secondary sidebar)

sessions/browser/paneCompositePartService.ts

AgenticPaneCompositePartService

sessions/browser/media/style.css

Layout-specific styles

sessions/browser/parts/parts.ts

AgenticParts enum

sessions/browser/parts/titlebarPart.ts

Titlebar part, MainTitlebarPart, AuxiliaryTitlebarPart, TitleService

sessions/browser/parts/sidebarPart.ts

Sidebar part (with footer and macOS traffic light spacer)

sessions/browser/parts/chatBarPart.ts

Chat Bar part

sessions/browser/parts/auxiliaryBarPart.ts

Auxiliary Bar part (with run script dropdown)

sessions/browser/parts/panelPart.ts

Panel part

sessions/browser/parts/projectBarPart.ts

Project Bar part (folder entries, icon customization)

sessions/contrib/configuration/browser/configuration.contribution.ts

Sets workbench.editor.useModal to 'all' for modal editor overlay

sessions/contrib/sessions/browser/sessionsTitleBarWidget.ts

Title bar widget and session picker

sessions/contrib/chat/browser/runScriptAction.ts

Run script split button for titlebar

sessions/contrib/accountMenu/browser/account.contribution.ts

Account widget for sidebar footer

sessions/electron-browser/parts/titlebarPart.ts

Desktop (Electron) titlebar part

5. Testing Changes

After modifying layout code:

• Verify the build compiles without errors via the VS Code - Build task

• Ensure the grid structure matches the spec's visual representation

• Confirm part visibility toggling works correctly (show/hide/maximize)

• Test that editors open in the ModalEditorPart overlay and that it closes properly

• Verify sidebar footer renders with account widget