[BUG] Web (claude.ai/code): split-pane view stopped rendering ~2026-07-09 — .dframe-pane-host no longer mounted; Cmd+click falls back to new tab

Summary

The split-pane / side-by-side session view in claude.ai/code (web) stopped rendering on ~2026-07-09 evening KST. It worked daily in the browser until then, and broke with no client-side change. The pane engine still ships in the web bundle and still persists state — only the render target was removed, so the feature silently degrades to opening a new browser tab.

Environment

• Google Chrome 150.0.0.0, macOS
• claude.ai/code in the browser (sessions hosted via a claude rc Remote Control bridge)
• Max plan

Steps to reproduce

1. Open claude.ai/code in Chrome and open a code session.
2. Cmd+click another session in the sidebar (or attempt any split gesture).

Expected: the second session opens in a pane beside the current one (behavior until 2026-07-09).
Actual: a new browser tab opens; no second pane ever renders.

Diagnosis (from reading the live web bundle)

In the current web bundle (index-BZuhUp9E.js):

• The split gesture is gated by a canSplit() that runs document.querySelector('.dframe-pane-host').
• On web, the dframe shell mounts .dframe-root / .dframe-sidebar / .dframe-content but no .dframe-pane-host — so canSplit() returns false and the handler falls through to window.open (the new tab).
• .dframe-pane-host / .dframe-pane-col appear only inside that query — the component that renders the pane host is no longer mounted (or shipped) in the web build.
• Meanwhile the pane store (desktop-frame.paneStore.v1 → extraPanesByMode) and the full pane logic (addPane, movePane, setMaxColumns) still ship and still persist state across reloads: injecting a valid second-pane entry persists but never renders.

So the regression is precisely: the pane-host container stopped being rendered in the web dframe shell as of the ~2026-07-09 release. Everything else needed for panes still ships in the web bundle.

Notes

• Not a Remote Control / bridge issue — bridge healthy throughout; same behavior across sessions.
• No JS console errors (silent degradation).
• If this was an intentional desktop-only migration rather than an accidental regression, please treat this as a formal request to restore web split panes — this was a daily-driver workflow in the browser, and the silent removal cost significant diagnostic effort (see also #76143 for a same-day report of a silent behavior change).
• Anthropic support conversation ID: 215475025490321 (escalated to human support with this diagnosis).