fix(cli): handle tmux false positive background detection (#27572)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Gal Zahavi <38544478+galz10@users.noreply.github.com>

packages/cli/src/ui/utils/terminalCapabilityManager.test.ts

@@ -117,6 +117,51 @@ describe('TerminalCapabilityManager', () => {
     expect(manager.getTerminalBackgroundColor()).toBe('#00ff00');
   });
 
+  it('should ignore #ffffff in tmux as it is a common false positive', async () => {
+    const manager = TerminalCapabilityManager.getInstance();
+    vi.spyOn(manager, 'isTmux').mockReturnValue(true);
+
+    const promise = manager.detectCapabilities();
+
+    // Simulate OSC 11 response for white
+    stdin.emit('data', Buffer.from('\x1b]11;rgb:ffff/ffff/ffff\x1b\\'));
+    // Complete detection with DA1
+    stdin.emit('data', Buffer.from('\x1b[?62c'));
+
+    await promise;
+    expect(manager.getTerminalBackgroundColor()).toBeUndefined();
+  });
+
+  it('should not ignore #ffffff when NOT in tmux', async () => {
+    const manager = TerminalCapabilityManager.getInstance();
+    vi.spyOn(manager, 'isTmux').mockReturnValue(false);
+
+    const promise = manager.detectCapabilities();
+
+    // Simulate OSC 11 response for white
+    stdin.emit('data', Buffer.from('\x1b]11;rgb:ffff/ffff/ffff\x1b\\'));
+    // Complete detection with DA1
+    stdin.emit('data', Buffer.from('\x1b[?62c'));
+
+    await promise;
+    expect(manager.getTerminalBackgroundColor()).toBe('#ffffff');
+  });
+
+  it('should NOT ignore other colors in tmux', async () => {
+    const manager = TerminalCapabilityManager.getInstance();
+    vi.stubEnv('TMUX', '1');
+
+    const promise = manager.detectCapabilities();
+
+    // Simulate OSC 11 response for grey
+    stdin.emit('data', Buffer.from('\x1b]11;rgb:8888/8888/8888\x1b\\'));
+    // Complete detection with DA1
+    stdin.emit('data', Buffer.from('\x1b[?62c'));
+
+    await promise;
+    expect(manager.getTerminalBackgroundColor()).toBe('#888888');
+  });
+
   it('should detect Terminal Name', async () => {
     const manager = TerminalCapabilityManager.getInstance();
     const promise = manager.detectCapabilities();

packages/cli/src/ui/utils/terminalCapabilityManager.ts

@@ -161,9 +161,21 @@ export class TerminalCapabilityManager {
               match[2],
               match[3],
             );
-            debugLogger.log(
-              `Detected terminal background color: ${this.terminalBackgroundColor}`,
-            );
+
+            // Heuristic: tmux 3.5+ may report #ffffff when it doesn't know the
+            // actual host terminal color (e.g. over mosh). We ignore this specific
+            // fallback value to prevent blinding the user with a light theme in a
+            // likely dark terminal.
+            if (this.terminalBackgroundColor === '#ffffff' && this.isTmux()) {
+              debugLogger.log(
+                'Ignored #ffffff background in tmux (common false positive over mosh).',
+              );
+              this.terminalBackgroundColor = undefined;
+            } else {
+              debugLogger.log(
+                `Detected terminal background color: ${this.terminalBackgroundColor}`,
+              );
+            }
           }
         }