Split right-panel panes have no close action when their only tab is fixed

[SawyerHood]

Summary

The right panel lets a user split a fixed tab, such as Info or Diff, into a new pane. The new pane has no close control because the fixed tab has no close button. The user can merge panes by dragging the tab back, but the interface does not show this action.

Versions and environment

• bb 0.39.0 from npm, which was the latest release during the test
• Ubuntu 26.04 LTS, x86_64
• Chromium web client connected to the local bb server
• Managed worktree environment env_cuscps8vbh
• Source check on main at 2cfd2b5df57daeed51ea544b54a8869bbec1c478

Steps to reproduce

1. Open a thread that has the Info and Diff fixed tabs.
2. Open the right panel.
3. Drag the Diff tab into the panel body.
4. Release the tab when the Split bottom target appears.
5. Try to close only the new Diff pane.

Smallest case: two fixed tabs are sufficient. A file tab does not have the same problem because it has a close button.

Expected vs actual

Expected:

Each split pane has a direct close action. This action also works when the pane contains only a fixed tab.

Actual:

The split has two panes, but neither pane has a Close pane control. The fixed Info and Diff tabs also have no close button. The only visible hide control closes the complete right panel and keeps the saved split.

Browser inspection after the split:

split panes: 2
buttons named "Close pane": 0
tab close buttons: 0
buttons named "Hide right panel": 1

Evidence

• Screenshot and investigation thread
• The split pane context sets onRequestClose to null: SidebarSplitContainer.tsx.
• Fixed tabs get select and drag handlers, but they get no close handler: ThreadSecondaryPanel.tsx.

What you ruled out

• This is not The thread right sidebar cannot split multiple panels #1841. That issue added right-panel splits but did not cover a close action for fixed-tab panes.
• This is not Sidebar panels as tabs and splits in the main view #1337. That issue covers moves from the sidebar into the main view.
• Dragging the fixed tab onto the center of the other pane merges the panes. This is a non-obvious workaround.
• Hiding and showing the complete right panel does not remove the saved split.
• Searches of open and closed issues found no report for this close-action gap.

Suggested priority and effort

Medium priority and low-to-medium effort. The problem affects each user who splits a fixed tab. It causes no data loss, but the workaround is not clear.

AGENT GENERATED