[Bug] Shift+Enter only inserts single newline instead of multiple

Bug Description
I can't insert more than one newline with shift+enter. This used to work. Nothing in my config has changed.

Environment Info

• Platform: darwin
• Terminal: tmux
• Version: 2.1.39
• Feedback ID: dc754cf6-0f20-4c10-9681-9323d2dc5764

Errors

[{"error":"Error: NON-FATAL: Lock acquisition failed for /Users/azolotov/.local/share/claude/versions/2.1.39 (expected in multi-process scenarios)\n    at wVR (/$bunfs/root/claude:3626:2098)\n    at DjA (/$bunfs/root/claude:3626:1209)\n    at processTicksAndRejections (native:7:39)","timestamp":"2026-02-11T22:01:13.184Z"}]