Keep the site preview webview warm with a contained surface

[shaunandrews]

Related issues

• Part of the window-resizing feature (integration branch agentic/window-resizing-fixes).

How AI was used in this PR

Built with Claude Code: hoisted the duplicated preview-surface wrapper into one place and standardized its visibility state. The diff was reviewed and typecheck confirmed.

Proposed Changes

• Keeps the site-preview webview mounted and warm while collapsed by rendering it inside a single contained surface that toggles visibility/interactivity, instead of two separate wrappers (one inside the Electron webview path, one around the iframe fallback).
• Collapses the surface's visibility state to a single active flag, removing the active/collapsed polarity flip between the two render paths, and switches the class composition to clsx to match the rest of apps/ui.

Testing Instructions

1. Open the agentic UI and open a site preview (Electron webview path).
2. Toggle the preview open/closed — the preview hides/shows and the webview keeps its state (stays warm; no reload).
3. If testing the non-Electron iframe fallback, confirm the same hide/show behavior.
4. Verify both light and dark schemes look correct.

Stack

Part of the window-resizing feature. Two independent PRs, both targeting agentic/window-resizing-fixes, mergeable in any order:

• Refactor panel resizing onto a shared drag primitive and a single clamp #3850 — Refactor panel resizing (shared primitive + single clamp)
• This PR — Keep the preview webview warm with a contained surface

Review just this PR's changes: compare agentic/window-resizing-fixes...warm-preview-surface (three dots).

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

🤖 Generated with Claude Code