fix(tui): disable fast-echo bypass inside tmux to prevent cursor drift

ui-tui/src/__tests__/textInputFastEcho.test.ts

@@ -178,6 +178,26 @@ describe('supportsFastEchoTerminal', () => {
     expect(supportsFastEchoTerminal({ TERM_PROGRAM: 'Apple_Terminal' } as NodeJS.ProcessEnv)).toBe(false)
   })
 
+  it('disables fast-echo inside tmux', () => {
+    expect(supportsFastEchoTerminal({ TMUX: '/tmp/tmux-1000/default,1234,0' } as NodeJS.ProcessEnv)).toBe(false)
+    expect(supportsFastEchoTerminal({ TMUX: '/private/tmp/tmux-501/default' } as NodeJS.ProcessEnv)).toBe(false)
+  })
+
+  it('tmux wins over Termux fast-echo opt-in', () => {
+    expect(
+      supportsFastEchoTerminal({
+        TMUX: '/tmp/tmux-1000/default,1234,0',
+        HERMES_TUI_TERMUX_FAST_ECHO: '1',
+        TERMUX_VERSION: '0.118.0'
+      } as NodeJS.ProcessEnv)
+    ).toBe(false)
+  })
+
+  it('keeps fast-echo enabled when TMUX is empty or unset', () => {
+    expect(supportsFastEchoTerminal({ TMUX: '' } as NodeJS.ProcessEnv)).toBe(true)
+    expect(supportsFastEchoTerminal({ TERM_PROGRAM: 'vscode' } 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)

ui-tui/src/components/textInput.tsx

@@ -359,6 +359,13 @@ export function supportsFastEchoTerminal(env: NodeJS.ProcessEnv = process.env):
     return false
   }
 
+  // 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) {
+    return false
+  }
+
   // Termux terminals are especially sensitive to bypass-path cursor drift and
   // stale paints at soft-wrap boundaries on tall/narrow viewports. Keep this
   // off by default in Termux mode; allow explicit opt-in for local debugging.