tmux/Bug/Vim Right Pane Insert Shows Up in Left Pane

• Problem Summary

  • In a tmux + Vim/Neovim workflow running inside iTerm2 on macOS, keyboard input typed in the right pane is visually rendered in the left pane.
  • This is not synchronize-panes (confirmed off).
  • The issue makes Vim nearly unusable because typed characters appear in the wrong pane despite focus being correct.

• Environment

  • macOS
  • iTerm2
  • tmux 3.6a
  • $TERM outside tmux: xterm-256color
  • $TERM inside tmux: tmux-256color
  • synchronize-panes: off
  • paste mode in Vim: off
  • assume-paste-time: 1
  • Using Vim or Neovim in split panes

• Observed Behavior

  • Two panes in tmux.
  • Vim running in right pane.
  • Insert mode active in right pane.
  • Characters typed appear in left pane display buffer.
  • Cursor visually remains in right pane.
  • No synchronize-panes enabled.
  • This suggests a terminal rendering desynchronization, not actual input duplication.

• What Is Not the Cause

  • Not tmux synchronize-panes.
  • Not basic $TERM mismatch.
  • Not Vim paste mode.
  • Not basic keybinding duplication.
  • Not shell prompt misbehavior.

• Most Likely Root Cause

  • Terminal focus-event or extended key negotiation desync between:
    • tmux ≥ 3.4 (3.6a in use)
    • iTerm2
    • Alternate screen apps (Vim)
  • tmux 3.6 tightened terminal feature negotiation, including:
    • Focus events
    • Extended keys
    • Bracketed paste
    • Terminal passthrough
    • Alternate screen behavior
  • iTerm2 partially supports some of these but can enter a bad state where:
    • Pane focus tracking is wrong
    • Cursor rendering and input routing diverge
    • Escape sequences leak across panes
  • The symptom (input displayed in wrong pane but not truly sent there) matches known focus-event and extended-key glitches.

• Hypothesis Ranking

  • focus-events enabled + iTerm2 bug
  • extended-keys negotiation bug
  • allow-passthrough misbehavior
  • alternate-screen rendering issue
  • iTerm2 session-level focus reporting

• Reproduction Isolation Plan

  • Step 1 — Clean tmux

    • tmux -L clean -f /dev/null

    • No config.
    • Split panes.
    • Run Vim in right pane.
    • Test input.
    • If problem disappears → configuration issue.
    • If problem persists → iTerm2 issue.

  • Step 2 — Disable tmux advanced features

    • Add to .tmux.conf:

    • set -g focus-events off
      set -g extended-keys off
      set -g allow-passthrough off
      set -g assume-paste-time 0
      set -g mouse off

    • Restart tmux fully:

    • tmux kill-server

    • Test again.

  • Step 3 — Disable alternate screen

    • Temporarily:

    • set -g alternate-screen off

    • If issue disappears, rendering negotiation is failing.

  • Step 4 — iTerm2 Settings Audit

    • In iTerm2:
    • Preferences → Advanced:
      • Disable:
        • Report Terminal Focus
        • Allow session to receive focus events
        • Applications may access clipboard
    • Preferences → Profiles → Terminal:
      • Uncheck "Report terminal focus"
    • Restart iTerm2 completely.

  • Step 5 — Force terminal compatibility downgrade

    • In .tmux.conf:

    • set -g default-terminal "screen-256color"

    • Instead of:

    • tmux-256color

    • Restart and test.
    • If problem disappears, terminfo capability mismatch is the trigger.

• Deep Diagnostic Commands

  • Run inside tmux:

  • tmux info
    tmux show -g
    tmux show-window-options -g
    infocmp tmux-256color

  • Run outside:

  • infocmp xterm-256color

  • Compare:
    • Ms
    • Tc
    • Focus capability
    • Smkx / Rmkx
    • XT
    • RGB
  • Look for capability mismatch.

• Underlying Technical Explanation

  • tmux tracks pane focus and rendering independently from terminal focus.
  • If:
    • Focus events are misreported,
    • Or extended key mode negotiation is incomplete,
    • Or alternate screen state desyncs,
  • then tmux may:
    • Route input to correct pane internally,
    • But terminal rendering buffer may attach to wrong pane,
    • Causing characters to visually appear in another pane.
  • This is a rendering synchronization fault, not input duplication.

• Recommended Fix Order

  • Disable focus-events.
  • Disable extended-keys.
  • Downgrade default-terminal to screen-256color.
  • Disable alternate-screen.
  • Disable focus reporting in iTerm2.
  • Clean tmux session test.

• Most Probable Final Fix

  • One of these lines resolving it:

  • set -g focus-events off

  • or

  • set -g default-terminal "screen-256color"

  • or disabling focus reporting in iTerm2.

• Next Steps

  • Investigate rendering desync between tmux 3.6a and iTerm2 involving focus-events, extended-keys, and alternate-screen negotiation.
  • Reproduce in clean tmux instance.
  • Systematically disable:
    • focus-events
    • extended-keys
    • allow-passthrough
    • alternate-screen
  • Test with downgraded default-terminal.
  • Determine whether issue is configuration-induced or iTerm2 feature negotiation incompatibility.