Split panel button silently duplicates sessions causing crashes and data corruption

Bug Report — High Priority

Problem

The VS Code Claude Code panel has a "split view" button (the split rectangle icon) positioned between the sparkle (new chat) and lock (permissions) buttons. Clicking it silently spawns a duplicate Claude process attached to the same session file with no warning whatsoever.

This causes:

1. Multiple processes writing to the same JSONL session file — confirmed via ps aux | grep resume showing 3 processes on the same session ID
2. Session corruption leading to "Prompt is too long" errors
3. Sessions appearing to swap between panels — because two panels are now fighting over the same session
4. Complete loss of user's working context — the user cannot tell which panel is the "real" one

Why This Is So Damaging

• The button has no confirmation dialog
• The button has no tooltip warning about what it does
• It's positioned right next to commonly used buttons (new chat, permissions) making accidental clicks inevitable
• The user gets zero feedback that anything happened — the duplicate just silently starts
• The damage (session corruption, crashes) only manifests later, making it nearly impossible to diagnose
• Users with 4-5 panels open will inevitably hit this by accident

Real-World Impact (today, single user)

A user lost an entire day of productivity:

• 10+ session crashes in a single work stream
• 3 zombie processes attached to one session (PIDs confirmed)
• Sessions swapping between panels mid-edit
• Ghost session files appearing that the user never created
• Multiple handoff documents had to be manually created to recover work
• Root cause traced back to accidental split panel button presses

Recommendation

1. Remove the split button entirely — splitting a Claude session makes no sense. Each session is a single conversation thread.
2. Or at minimum: add a confirmation dialog ("This will create a duplicate view of this session. Are you sure?")
3. Or: make split view read-only (view the conversation but can't send messages)
4. Or: move the button far away from the other controls

Related Issues

• #25295 (sessions spontaneously split and swap between panels)
• #25274 (tool results embed full file content causing prompt-too-long)
• #24709 (session ID not visible in tab titles)

Environment

• Claude Code v2.1.39 (VS Code extension, macOS)
• macOS Darwin 25.2.0