Clipboard copy fails in tmux when allow-passthrough is off #25253

Summary

OpenCode's TUI clipboard helper always wraps OSC 52 in the tmux DCS passthrough envelope whenever TMUX or STY is present:

const passthrough = process.env[\"TMUX\"] || process.env[\"STY\"]
const sequence = passthrough ? `\x1bPtmux;\x1b${osc52}\x1b\\` : osc52

That breaks copy actions in tmux sessions where allow-passthrough off is required. In that topology, tmux's DCS passthrough is intentionally blocked, so OpenCode copy surfaces no longer reach the client clipboard.

Reproduction

1. Start a tmux session on a Linux host over SSH.
2. Keep set -g set-clipboard on and set -g allow-passthrough off in tmux.
3. Run opencode inside tmux.
4. Try any clipboard action such as:
   • copying a selected message
   • right-click selection copy
   • Copy session transcript
   • any other Clipboard.copy(...) path

Observed

Clipboard actions do not update the local terminal clipboard.

Expected

OpenCode should support a non-passthrough clipboard mode for tmux sessions where allow-passthrough is off, for example:

• a raw OSC 52 mode / env override, or
• tmux-aware fallback behavior that does not require DCS passthrough.

Why this matters

allow-passthrough off is a legitimate tmux hardening/workaround state. If OpenCode assumes tmux passthrough is always available whenever TMUX exists, clipboard support silently regresses in exactly the environments where tmux users need a safer topology.

Traceability

• Tools used: OpenCode