Description

When the sidebar is collapsed and the workspace rail (Workspace Files) is visible (viewport > 1120px), clicking the session picker button (aria-controls="chat-session-picker-sidebar") shows the session picker popover, but it is partially covered by the chat content area and completely unusable:

• Cannot click any session option
• Cannot scroll the session list
• Cannot type in the search input
• The chat card paints on top of the picker

Steps to Reproduce

1. Start the OpenClaw Control UI (openclaw gateway)
2. Navigate to a chat session with the sidebar collapsed
3. Ensure the viewport is wide enough (>1120px) for the Workspace Files rail to render
4. Click the session picker button in the collapsed sidebar
5. Try to interact with the session picker popover

Observed Behavior

The session picker popover appears behind the chat content card. It is not clickable, not scrollable, and the search input cannot receive focus.

Expected Behavior

The session picker popover should render above all other content, be fully interactive, scrollable, and searchable.

Additional Context

• The bug does not occur when the viewport is narrow (<1120px) and the Workspace Files rail is hidden (via display: none)
• The root cause is a CSS z-index layering issue between .shell-nav and .content--chat grid siblings, combined with an undefined --z-dropdown CSS variable