Agent Mode: Match Chat Mode input auto-sizing and add new-chat expand/collapse

[marksftw]

Summary

Bring Agent Mode's prompt input sizing behavior into parity with Chat Mode:

1. Automatically grow and shrink the Agent composer as its content changes on both the new-chat screen and existing chat detail screens.
2. Add Chat Mode's large Expand/Collapse behavior to the Agent Mode new-chat screen only.
3. Persist the Agent expansion preference independently from Chat Mode.

Chat Mode is the behavioral reference. This should be implemented as an Agent-focused change; avoid modifying Chat Mode unless it is genuinely unavoidable.

Current behavior

Agent Mode uses the same AgentComposer on both surfaces:

• the empty/new-chat state
• the fixed composer beneath an existing agent timeline

The composer already reuses ChatComposerSurface and CHAT_COMPOSER_TEXTAREA_CLASS, including the normal minimum and maximum height styles. However, its textarea remains at its initial height as text wraps or new lines are added.

The relevant difference is behavioral:

• Chat Mode holds a textarea ref and recalculates its inline height from scrollHeight whenever the input changes.
• AgentComposer renders a one-row textarea but has no equivalent auto-sizing behavior.
• A CSS max-height constrains growth but does not cause a textarea to grow by itself.

Chat Mode also has a large composer toggle on its new-chat screen. Agent Mode has no equivalent.

Desired behavior

Automatic sizing on both Agent Mode screens

The Agent textarea should behave like the normal Chat Mode textarea:

• Start at the existing compact height.
• Grow as entered or pasted text wraps onto additional lines.
• Shrink again when lines are removed or the input is cleared.
• Stop growing at the same normal maximum height used by Chat Mode.
  • Normal layouts: 200px.
  • landscape-short: preserve the effective 100px cap from the shared styling.
• Once capped, keep the composer stable and allow the textarea itself to scroll vertically.
• Avoid page or timeline layout overflow while resizing.

This behavior must apply to:

• Agent Mode's new-chat/empty-session composer.
• The composer on an existing agent chat detail screen, including a previously loaded session.

Large Expand/Collapse mode on Agent new chat

Add the same large composer affordance used by Chat Mode to the Agent Mode new-chat screen.

When expanded:

• Use the same overall sizing behavior as Chat Mode (approximately 70vh, capped at 800px).
• Let the textarea fill the available vertical space.
• Keep the Agent-specific control row—model, permission mode, project selector, and send button—anchored at the bottom of the composer.
• Use the corresponding Expand/Shrink icon treatment, transition, placement, and accessible toggle labeling from Chat Mode.
• Hide the new-chat heading and privacy message while expanded, matching Chat Mode.
• Apply the same wider expanded layout treatment as Chat Mode.

When collapsed, return to normal auto-sizing behavior.

The manual Expand/Collapse control should appear only on the new-chat/empty-session screen. It should not appear on existing chat detail screens.

Persistence and session transitions

Follow Chat Mode's persistence behavior, but keep the two modes independent:

• Store the Agent preference separately from chatFullscreen; agentFullscreen is the suggested key.
• Reloading or returning to a new Agent chat should restore the last Agent expansion preference.
• Chat Mode's expansion preference must not change Agent Mode, and vice versa.
• Sending the first prompt from an expanded new-chat screen should transition to the normal compact detail composer.
• The saved expanded preference should remain intact, so the next new/empty Agent session opens expanded.

Technical direction

Prefer the smallest Agent-only implementation:

• Add the auto-sizing ref/effect once inside AgentComposer, since that component already serves both new-chat and detail surfaces.
• Add Agent-specific expanded state and persistence at the level that survives the new-chat-to-detail transition.
• Pass explicit expansion/toggle props only where the new-chat composer needs them.
• Continue reusing the existing shared ChatComposerSurface and textarea styling.
• Adjust EmptyAgentState layout as needed so the expanded composer can match Chat Mode's height, width, and hidden heading/privacy behavior.

Do not turn this into a universal Chat/Agent composer rewrite. The action rows, locking rules, attachments, and send behavior differ substantially between the modes. A small shared primitive is acceptable if it is clearly low-risk, but extracting or rewriting Chat Mode is not required and should be avoided if the Agent implementation can remain self-contained.

Preserve existing behavior

This change must not alter:

• Desktop Enter-to-send and Shift+Enter-for-newline behavior.
• Existing compact-layout/mobile keyboard behavior.
• Agent model, permission mode, and project selection behavior.
• Send, cancel, initialization, loading, and disabled states.
• Timeline scrolling or session switching behavior.
• Chat Mode's current appearance, auto-sizing, expansion behavior, or persisted preference.

Acceptance criteria

• A one-line Agent prompt retains the current compact composer height.
• Typing or pasting multiline content grows the new-chat Agent textarea line by line.
• Typing or pasting multiline content grows the existing-chat Agent textarea line by line.
• Deleting content shrinks both Agent composer variants back toward their compact height.
• Normal growth caps at the same effective heights as Chat Mode and then scrolls internally.
• Starting, clearing, cancelling, or switching sessions does not leave a stale inline height.
• Agent new chat has a working Expand/Collapse toggle matching Chat Mode's large composer behavior.
• Expanded Agent new chat keeps all Agent controls usable and pinned to the bottom.
• The heading and privacy message are hidden while expanded and restored when collapsed.
• Existing Agent chat detail screens do not show the manual Expand/Collapse control.
• Sending from expanded new chat transitions to the compact detail composer.
• The Agent expanded preference survives reloads/new sessions under its own persisted key.
• Agent and Chat expansion preferences do not affect each other.
• Keyboard submission and newline behavior remain unchanged.
• Compact and landscape-short layouts remain usable without viewport overflow.
• Chat Mode has no visual or behavioral regression.

[AnthonyRonning]

if you want to test this, here it is: #632