[App] Composer shadow overlays conversation content while scrolling on macOS

Environment

• App: ChatGPT/Codex desktop
• App version: 26.730.61639
• App build: 6234
• macOS: 26.3 (25D125)
• Architecture: Apple Silicon (arm64)
• Theme: Light
• UI language: French

What issue are you seeing?

While scrolling a conversation, the fixed composer casts an oversized translucent white/gray shadow or backdrop upward over the conversation content. Text and card borders become faded or partially obscured as they pass behind this layer.

This is specifically a shadow overlap during scrolling, rather than a persistent translucent layer below the composer.

Steps to reproduce

1. Open a conversation containing enough content to scroll.
2. Scroll until conversation content passes near the fixed composer at the bottom of the viewport.
3. Observe the broad shadow layer extending upward from the composer and covering the scrolled content.

Expected behavior

The composer shadow should provide subtle visual separation without covering readable conversation content. The shadow should be constrained appropriately, or the scrollable content should end above the composer.

Actual behavior

A large translucent shadow overlays the content and reduces its legibility.

Additional information

Related but distinct from #24537: that report concerns a translucent layer below the composer on an Intel Mac, while this report concerns the composer shadow extending upward over scrolled content on Apple Silicon.

<img width="620" height="167" alt="Image" src="https://github.com/user-attachments/assets/ee19b82b-b31a-4a3b-a324-009608707c48" />