diff --git a/tests/tools/test_computer_use_cua_backend_linux.py b/tests/tools/test_computer_use_cua_backend_linux.py
new file mode 100644
index 000000000000..a7d6722f96d9
--- /dev/null
+++ b/tests/tools/test_computer_use_cua_backend_linux.py
@@ -0,0 +1,153 @@
+"""Regression tests for Linux/X11 capture target selection (#58026)."""
+
+from __future__ import annotations
+
+from unittest.mock import patch
+
+# Tied z_index=0 fixture from #58026 (ding ahead of real terminals).
+ISSUE_58026_WINDOWS = [
+    {
+        "app_name": "ding",
+        "pid": 4294,
+        "window_id": 33554439,
+        "title": "Desktop Icons 1",
+        "is_on_screen": True,
+        "z_index": 0,
+    },
+    {
+        "app_name": "",
+        "pid": 1816017,
+        "window_id": 60817412,
+        "title": "zcode",
+        "is_on_screen": True,
+        "z_index": 0,
+    },
+    {
+        "app_name": "",
+        "pid": 1877178,
+        "window_id": 84043449,
+        "title": "xr@10:~/hermes",
+        "is_on_screen": True,
+        "z_index": 0,
+    },
+    {
+        "app_name": "",
+        "pid": 1877178,
+        "window_id": 84065715,
+        "title": "HERMES-CU",
+        "is_on_screen": True,
+        "z_index": 0,
+    },
+]
+
+
+def _normalized_windows(raw=ISSUE_58026_WINDOWS):
+    from tools.computer_use.cua_backend import _ingest_windows
+
+    return _ingest_windows(raw)
+
+
+def test_parse_xprop_net_active_window_standard_output():
+    from tools.computer_use.cua_backend import _parse_xprop_net_active_window
+
+    raw = "_NET_ACTIVE_WINDOW(WINDOW): window id # 0x503000b\n"
+    assert _parse_xprop_net_active_window(raw) == 0x503000b
+
+
+def test_parse_xprop_net_active_window_bare_hex_fallback():
+    from tools.computer_use.cua_backend import _parse_xprop_net_active_window
+
+    assert _parse_xprop_net_active_window("active=0xABcdef01") == 0xABCDEF01
+
+
+def test_parse_xprop_net_active_window_rejects_unparseable():
+    from tools.computer_use.cua_backend import _parse_xprop_net_active_window
+
+    assert _parse_xprop_net_active_window("") is None
+    assert _parse_xprop_net_active_window("_NET_ACTIVE_WINDOW(WINDOW): none") is None
+    assert _parse_xprop_net_active_window("window id # not-a-hex") is None
+
+
+def test_default_capture_prefers_x11_active_window_when_z_index_tied():
+    from tools.computer_use.cua_backend import _select_capture_target
+
+    windows = _normalized_windows()
+
+    with patch("tools.computer_use.cua_backend.sys.platform", "linux"), patch(
+        "tools.computer_use.cua_backend._linux_x11_active_window_id",
+        return_value=84043449,
+    ):
+        target = _select_capture_target(windows, app_requested=False)
+
+    assert target["title"] == "xr@10:~/hermes"
+    assert target["window_id"] == 84043449
+
+
+def test_default_capture_falls_back_to_list_order_when_active_window_unknown():
+    from tools.computer_use.cua_backend import _select_capture_target
+
+    windows = _normalized_windows()
+
+    with patch("tools.computer_use.cua_backend.sys.platform", "linux"), patch(
+        "tools.computer_use.cua_backend._linux_x11_active_window_id",
+        return_value=None,
+    ):
+        target = _select_capture_target(windows, app_requested=False)
+
+    # Without informative z-order or active-window, keep list order (caller
+    # sorts higher-z frontmost; here all tied so first on-screen wins).
+    assert target["window_id"] == 33554439
+    assert target["title"] == "Desktop Icons 1"
+
+
+def test_default_capture_keeps_higher_z_index_when_ordering_informative():
+    """Active-window fallback must not override a real frontmost z_index."""
+    from tools.computer_use.cua_backend import _select_capture_target
+
+    windows = _normalized_windows(
+        [
+            {
+                "app_name": "",
+                "pid": 1,
+                "window_id": 10,
+                "title": "back",
+                "is_on_screen": True,
+                "z_index": 1,
+            },
+            {
+                "app_name": "",
+                "pid": 2,
+                "window_id": 20,
+                "title": "front",
+                "is_on_screen": True,
+                "z_index": 5,
+            },
+        ]
+    )
+    # Mirror _load_windows: higher z_index is frontmost.
+    windows.sort(key=lambda w: w["z_index"], reverse=True)
+
+    with patch("tools.computer_use.cua_backend.sys.platform", "linux"), patch(
+        "tools.computer_use.cua_backend._linux_x11_active_window_id",
+        return_value=10,
+    ) as active:
+        target = _select_capture_target(windows, app_requested=False)
+
+    assert target["window_id"] == 20
+    assert target["title"] == "front"
+    active.assert_not_called()
+
+
+def test_explicit_app_capture_skips_active_window_fallback():
+    from tools.computer_use.cua_backend import _select_capture_target
+
+    windows = _normalized_windows()
+
+    with patch("tools.computer_use.cua_backend.sys.platform", "linux"), patch(
+        "tools.computer_use.cua_backend._linux_x11_active_window_id",
+        return_value=84043449,
+    ) as active:
+        target = _select_capture_target(windows, app_requested=True)
+
+    assert target["window_id"] == 33554439
+    active.assert_not_called()