Pane(fix[capture]): trim trailing whitespace consistently

why: Control mode and subprocess should return matching capture-pane output.
what:
- Trim trailing whitespace-only lines in control protocol and Pane.capture_pane
- Add regression fixtures for whitespace tails across engines

Author: tony

src/libtmux/_internal/engines/control_protocol.py

@@ -24,9 +24,9 @@
 
 
 def _trim_lines(lines: list[str]) -> list[str]:
-    """Remove trailing empty strings to mirror subprocess behaviour."""
+    """Remove trailing empty or whitespace-only lines to mirror subprocess behaviour."""
     trimmed = list(lines)
-    while trimmed and trimmed[-1] == "":
+    while trimmed and trimmed[-1].strip() == "":
         trimmed.pop()
     return trimmed
 

src/libtmux/pane.py

@@ -430,9 +430,9 @@ def capture_pane(
         output = self.cmd(*cmd).stdout
 
         def _trim(lines: list[str]) -> list[str]:
-            # Match engine trimming: remove only empty strings, not whitespace-only
+            # Match engine trimming: remove trailing empty or whitespace-only lines
             trimmed = list(lines)
-            while trimmed and trimmed[-1] == "":
+            while trimmed and trimmed[-1].strip() == "":
                 trimmed.pop()
             return trimmed
 

tests/test_control_mode_regressions.py

@@ -64,6 +64,14 @@ class AttachFixture(t.NamedTuple):
         ),
         id="many_blanks",
     ),
+    pytest.param(
+        TrailingOutputFixture(
+            test_id="whitespace_tail",
+            raw_lines=["line1", "   ", " ", ""],
+            expected_stdout=["line1"],
+        ),
+        id="whitespace_tail",
+    ),
 ]
 
 

tests/test_pane.py

@@ -96,6 +96,23 @@ def test_capture_pane(session: Session) -> None:
         assert pane_contents == expected_full
 
 
+@pytest.mark.engines(["subprocess", "control"])
+def test_capture_pane_trims_whitespace_tail(session: Session) -> None:
+    """capture-pane should drop trailing whitespace-only lines for all engines."""
+    pane = session.active_pane
+    assert pane is not None
+
+    pane.send_keys('printf "line1\\n   \\n"', literal=True, suppress_history=False)
+    wait_for_line(pane, lambda line: "line1" in line)
+
+    lines = pane.capture_pane()
+    assert lines
+    # The last line should not be empty/whitespace-only
+    assert lines[-1].strip() != ""
+    # Ensure the whitespace-only line was trimmed
+    assert "line1" in "\n".join(lines)
+
+
 def test_capture_pane_start(session: Session) -> None:
     """Assert Pane.capture_pane() with ``start`` param."""
     env = shutil.which("env")