From e52fffb607fe560604d5645f57d84d71d6c8b51e Mon Sep 17 00:00:00 2001
From: kshitijk4poor <82637225+kshitijk4poor@users.noreply.github.com>
Date: Fri, 19 Jun 2026 16:09:33 +0530
Subject: [PATCH] harden(tui): also disable fast-echo for tmux-flavored TERM
 (SSH-from-tmux)

TMUX is not forwarded over SSH, so a TUI launched on a remote host from
inside local tmux only sees TERM=tmux/tmux-256color with no TMUX var --
the cursor-drift bug still applies there. Extend supportsFastEchoTerminal()
to also fall back when TERM is tmux-flavored.

Deliberately scoped to tmux* only, NOT screen*: GNU screen sets the same
screen/screen-256color TERM and has no reported drift, so widening to
screen would disable the optimization for those users with no evidence of
a bug (matching the original PR's stated out-of-scope note).

Adds tests for tmux-flavored TERM (disabled) and screen/xterm TERM
(stays enabled) to guard against accidental widening.
---
 ui-tui/src/__tests__/textInputFastEcho.test.ts | 17 +++++++++++++++++
 ui-tui/src/components/textInput.tsx            | 11 ++++++++++-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/ui-tui/src/__tests__/textInputFastEcho.test.ts b/ui-tui/src/__tests__/textInputFastEcho.test.ts
index 03805aa38..98928d1ba 100644
--- a/ui-tui/src/__tests__/textInputFastEcho.test.ts
+++ b/ui-tui/src/__tests__/textInputFastEcho.test.ts
@@ -198,6 +198,23 @@ describe('supportsFastEchoTerminal', () => {
     expect(supportsFastEchoTerminal({ TERM_PROGRAM: 'vscode' } as NodeJS.ProcessEnv)).toBe(true)
   })
 
+  it('disables fast-echo when only a tmux-flavored TERM is present (SSH from tmux, no TMUX forwarded)', () => {
+    // OpenSSH forwards TERM but not TMUX, so a TUI on a remote host launched
+    // from inside local tmux sees TERM=tmux-256color with no TMUX var. The
+    // cursor-drift bug still applies, so fast-echo must stay off.
+    expect(supportsFastEchoTerminal({ TERM: 'tmux' } as NodeJS.ProcessEnv)).toBe(false)
+    expect(supportsFastEchoTerminal({ TERM: 'tmux-256color' } as NodeJS.ProcessEnv)).toBe(false)
+  })
+
+  it('does NOT disable fast-echo for screen-flavored TERM (GNU screen out of scope, no reported drift)', () => {
+    // GNU screen sets TERM=screen/screen-256color and has no reported drift.
+    // We must not widen the tmux guard to screen* and regress its perf.
+    expect(supportsFastEchoTerminal({ TERM: 'screen' } as NodeJS.ProcessEnv)).toBe(true)
+    expect(supportsFastEchoTerminal({ TERM: 'screen-256color' } as NodeJS.ProcessEnv)).toBe(true)
+    // And an unrelated 256color TERM must stay enabled.
+    expect(supportsFastEchoTerminal({ TERM: 'xterm-256color' } as NodeJS.ProcessEnv)).toBe(true)
+  })
+
   it('disables fast-echo by default in Termux mode', () => {
     expect(
       supportsFastEchoTerminal({ TERMUX_VERSION: '0.118.0', PREFIX: '/data/data/com.termux/files/usr' } as NodeJS.ProcessEnv)
diff --git a/ui-tui/src/components/textInput.tsx b/ui-tui/src/components/textInput.tsx
index ff6c9dad7..deb229146 100644
--- a/ui-tui/src/components/textInput.tsx
+++ b/ui-tui/src/components/textInput.tsx
@@ -362,7 +362,16 @@ export function supportsFastEchoTerminal(env: NodeJS.ProcessEnv = process.env):
   // tmux adds a PTY multiplexing layer that desyncs stdout.write() cursor
   // advances from its internal cursor model, causing cursor drift and ghost
   // whitespace under the fast-echo bypass path.
-  if ((env.TMUX ?? '').trim().length > 0) {
+  //
+  // `TMUX` catches the local case. It is NOT forwarded over SSH, so when the
+  // TUI runs on a remote host launched from inside local tmux we only see a
+  // tmux-flavored `TERM` (tmux sets `tmux`/`tmux-256color`); match that too so
+  // remote-over-tmux sessions still fall back to the safe render path. We
+  // deliberately do NOT match `screen*`: GNU screen sets the same TERM and has
+  // no reported drift, so widening to screen would disable the optimization for
+  // those users with no evidence of a bug.
+  const term = (env.TERM ?? '').trim().toLowerCase()
+  if ((env.TMUX ?? '').trim().length > 0 || term === 'tmux' || term.startsWith('tmux-')) {
     return false
   }