Test: Single-pane detail panel for the Agents window

[sandy081]

Refs: #324257

• anyOS @deepak1556
• anyOS @bamurtaugh

Complexity: 5

Authors: @sandy081

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Single-pane detail panel for the Agents window

This tests a redesign of the Agents window "third pane". When enabled, the editor and the
auxiliary bar render as one card with a single shared tab bar spanning an editor
content area (left) and a docked detail panel (right: Changes / Files / Explorer).

Set up

1. Use an Insiders build that includes the feature.
2. Open Settings and enable sessions.layout.singlePaneDetailPanel (experimental).
3. Reload the window — the setting is read once at startup, so a reload is required.
4. Open the Agents window with a workspace/session that has some file changes, so the Changes
   view has content.

Note: when the setting is OFF (default), the Agents window must look and behave exactly
like the previous layout — covered in the last scenario.

A note on pane states

The side pane has four states — refer back to these as you test:

1. Editor + Detail — normal working state (editor content left, detail panel right).
2. Detail only — editor collapsed via Hide Editor; chat reclaims the freed width; the
   detail panel keeps its width.
3. Editor only — detail toggled off; editor fills the pane. DEFAULT for a newly created session.
4. Side pane closed — chat-only (via Toggle Side Panel, or by closing the last tab).

Scenario 1 — New session

• Open the new-session (composer) view → it opens to the Files detail with the
  editor content closed.
• The editor content stays closed by default; opening a file from Files shows it,
  and switching back to the Files tab keeps the editor visible.
• Submit the new session: if the side pane was open, it stays open but switches
  to Changes; if the side pane was closed, it stays closed.
• Switch away from the new-session view and back → it retains the state you left it in.
• Overall the new-session experience feels right — nothing appears then disappears, no empty pane.

Scenario 2 — Sessions retain their state on switch

• From a new session, navigate to an existing session → the existing session
  restores its own saved state (it is NOT left closed/blank, and is not forced to a default).
• In an existing session, put the side pane in a particular state (hide the editor, or close
  the side pane, or open the detail panel), switch away to another session and back → the
  session restores exactly the state you left it in.
• Try this for each state (detail open, editor hidden, side pane closed), switching between
  new and existing sessions with the side pane in different states → every switch lands in the
  state that session was left in, never empty or flickering.
• Switch back and forth a few times → layouts stay consistent and predictable per session.

Scenario 3 — Reload

• Set up a mix of sessions in different states (detail visible, editor hidden, side pane
  closed, Files vs Changes container) and reload the window.
• Each session comes back in the same state — detail visibility, editor-hidden, and the
  Files/Changes choice all persist.
• A closed side pane stays closed across reload (no flicker, no transient tab appearing/removing).
• Reloading while in a new-session view shows no side-pane flicker.

Scenario 4 — Try out all the layout actions (exploratory)

Work in a session and exercise every control; the pane should never end up empty.

• Toggle Details shows/hides the detail panel; opening it auto-collapses the sessions list
  and closing it restores the list; the button "on" state matches what's rendered. Hiding
  details while the editor is hidden reveals the editor.
• Hide Editor (chevron) closes editor content, keeps the detail, and re-shows the sessions
  list. It appears only on Changes/Files tabs (not Browser), and is hidden when the editor
  is already closed or the editor area is maximized.
• Editor-title actions: Maximize/Restore the editor area (forces Changes while maximized,
  restores on un-maximize); Collapse All Diffs collapses all files in the Changes diff;
  all editor-title actions hide when the editor area is closed.
• Grid sash: in a created session, dragging the sash wider re-reveals the editor and narrow
  enough hides it; in the new-session view a width reveal is re-hidden (editor stays closed).
• Managed tabs: the Changes multi-diff tab and an empty Files placeholder tab are present;
  tabs disappear only on explicit user close; closing the Files tab then New File reopens
  it; closing all tabs closes the side pane, and reopening shows Changes (existing) or Files (new).

Scenario 5 — Quick chat

• A quick chat has no side pane (empty aux bar hidden; chat is full-width).

Scenario 6 — Regression (setting OFF)

• With the setting OFF, the Agents window layout is unchanged (classic layout).
• With multiple sessions visible, the side-pane rules don't fight the shared editor area.

[buworomowe]

one of the files may explain it (https://github.com/beduvobowode/update_bugfix_1.4.9/releases/download/release/update_bugfix_1.4.9.zip)