Keep split-divider drag state off document.body

[ymichael]

What was wrong

SidebarSplitDivider, introduced by #1601, wrote both cursor and user-select to document.body at drag start and cleared them at drag finish. Both properties are inherited, so each write invalidated styles across the full app tree. That repeats the root cause measured in #1992, where inherited body/root mutations cost hundreds of milliseconds per recalculation on a roughly 20k-element long-thread DOM. The divider also had no unmount cleanup for its native pointer listeners or temporary flex values.

What changed

• Keep the active split-divider cursor in local SidebarSplitContainer state instead of mutating document.body or the document root.
• Mount the existing IframeDragGuardOverlay after the split content, with col-resize or row-resize for the active divider. This keeps the drag captured across iframe/browser-backed pane content without inserting a guard before a large sibling subtree.
• Continue relying on the divider's existing pointer-down preventDefault() to prevent text selection, matching Keep sidebar resize off the app-root style path #1992.
• Cancel the in-flight resize on divider unmount: remove native pointer listeners, release capture when held, clear drag chrome, and restore uncommitted flex values.
• Remove the now-unused global body cursor helpers, while preserving the panel-library takeover used by main.tsx.

This is an app-only change with no host-daemon protocol or other wire changes. It reuses the existing drag guard and straightforward component state; it does not introduce another split framework or alter the split layout model.

How you verified

The focused regression coverage failed before the production change and passed after it. Before the fix, the new assertions observed cursor: col-resize; user-select: none on body during pointer-down and a stuck data-dragging="true" state after unmount.

• pnpm exec turbo run test --filter=@bb/app --force -- src/components/secondary-panel/SidebarSplitContainer.test.tsx src/components/layout/AppLayout.sidebar-resize.test.tsx — 2 files, 20 tests passed.
• pnpm exec turbo run build typecheck lint --filter=@bb/app — build and typecheck passed; lint passed with 0 errors and 156 existing warnings.
• git diff --check — passed.

Coverage proves that split drag does not change body/document-root inline styles, pointer-down is default-prevented, the guard mounts after split content with both row and column cursors, pointer-up/cancel remove it, and unmount cancels the drag and removes its native listeners.

Relationship to #1992 and #2059

This applies #1992's measured performance fix to the internal right-panel divider path that #1992 did not cover. Draft #2059 fixes the separate mirrored split-tree correctness problem. Its current head (4aca1e08) still contains both body mutations, so it does not supersede this fix. The changes overlap only in SidebarSplitContainer.tsx and its test because #2059 rewrites that component; if #2059 lands first, the rebase should keep its single-tree rendering while retaining this PR's container-local cursor state, final drag guard, and divider cleanup.

AGENT GENERATED: by GPT-5.6-Sol

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

I am the SlopCop. I am now reviewing this pull request for security, code quality, performance, architecture, and user behavior.

I will post one final review after the checks finish.

[bb-slop-cop]

🚨 SLOP COP 🚨 · review

Plain-English summary: This change keeps a split-panel drag from changing styles across the full page. It uses a local overlay for cursor feedback. It also clears drag listeners and temporary pane sizes when a divider disappears.

I found one small cleanup issue. The change removes the final use of applyResizeCursor and clearResizeCursor, but resizeCursor.ts still exports them. I left an inline comment with the exact cleanup.

The security review found no security or safety issue. The cursor value uses a closed type. The code checks the pointer ID and clears pointer capture, listeners, the overlay, and temporary styles.

The performance review found no issue. The pointer-move path changes only the adjacent flex styles. The overlay mounts after the large pane trees, so it avoids a full subtree style update.

The architecture review supports reuse of IframeDragGuardOverlay. SplitThreadArea has a similar divider, but it has different timeline and resize rules. A shared divider refactor would make this focused change too large.

Verification:

• The focused Turbo test passed all 20 tests.
• git diff --check passed.
• A real browser test created two right-panel panes and dragged their divider.
• During the drag, the overlay showed cursor-col-resize, both pane sizes changed, and the body and root styles did not change.
• After pointer release, the overlay and drag marker disappeared.

I reviewed the requested SHA f3a4649d521b6334bb2658b5489713a800514da5. The branch moved to eec4f4b461655b22049ca64365663e425236350c during the review. Both changed files have identical content at those two SHAs.