The thread right sidebar cannot split multiple panels

[brsbl]

Summary

The thread right sidebar can hold multiple tabs but can display only one panel at a time. Users cannot keep complementary context—such as a diff, terminal, browser, and file preview—visible together inside the sidebar, resize those regions, or preserve that arrangement across wide and compact layouts.

Implementation: PR #1601

Versions and environment

• Surface: thread right sidebar in the bb app
• Applies to wide desktop layouts and compact responsive drawer behavior
• Gap exists on main before 065902ab76895c7384bb8e4fabd1ead234bed8c2

Steps to reproduce

1. Open two or more right-sidebar tabs in a thread.
2. Try to place the active tab left, right, above, or below another sidebar tab.
3. Try to resize, recombine, full-screen, restore, and retain that split between wide and compact layouts.

Expected vs actual

Expected: the sidebar can persist a split tree, align tab headers with pane bodies, target the focused pane for add-tab actions, and show one active drawer on compact viewports without discarding the saved wide layout.

Actual: the sidebar exposes one unsplit active tab surface.

Evidence

• The implementation introduces the split host in SidebarSplitContainer.
• Persistence, commands, resizing, compact retention, browser/terminal ownership, and real-interface QA are covered in PR #1601.

What you ruled out

• Not Sidebar panels as tabs and splits in the main view #1337: that proposal tears sidebar panels into the main workspace; this capability keeps splits inside the existing right sidebar.
• Not split-thread layout: this subdivides one thread's secondary panel.
• No exact open or closed issue matched the sidebar split scope, PR URL, or originating thread.

Suggested priority and effort

Medium priority, high effort: materially improves multi-context workflows but touches persistent layout, geometry, browser/terminal ownership, desktop IPC, responsive behavior, and recovery states.

Originating implementation thread: thr_4rr623umv4

AGENT GENERATED: by GPT-5.6-Sol