Pane(feat[split]): add percentage parameter for split-window

why: split-window supports -p for percentage-based sizing which is more
intuitive than the absolute cell count provided by the existing size parameter.
what:
- Add percentage (-p) parameter to Pane.split(), mutually exclusive with size
- Validate that size and percentage are not both specified
- Add tests for percentage split and mutual exclusion

Author: tony

src/libtmux/pane.py

@@ -876,6 +876,7 @@ def split(
         zoom: bool | None = None,
         shell: str | None = None,
         size: str | int | None = None,
+        percentage: int | None = None,
         environment: dict[str, str] | None = None,
     ) -> Pane:
         """Split window and return :class:`Pane`, by default beneath current pane.
@@ -903,7 +904,12 @@ def split(
             is useful for long-running processes where the closing of the
             window upon completion is desired.
         size: int, optional
-            Cell/row or percentage to occupy with respect to current window.
+            Cell/row count to occupy with respect to current window.
+        percentage: int, optional
+            Percentage (0-100) of the window to occupy (``-p`` flag).
+            Mutually exclusive with *size*.
+
+            .. versionadded:: 0.45
         environment: dict, optional
             Environmental variables for new pane. Passthrough to ``-e``.
 
@@ -969,9 +975,16 @@ def split(
         else:
             tmux_args += tuple(PANE_DIRECTION_FLAG_MAP[PaneDirection.Below])
 
+        if size is not None and percentage is not None:
+            msg = "Cannot specify both size and percentage"
+            raise ValueError(msg)
+
         if size is not None:
             tmux_args += (f"-l{size}",)
 
+        if percentage is not None:
+            tmux_args += (f"-p{percentage}",)
+
         if full_window_split:
             tmux_args += ("-f",)
 

tests/test_pane.py

@@ -712,3 +712,29 @@ def test_display_message_flags(
         assert result is not None
         output = "\n".join(result)
         assert expected_in_output in output
+
+
+def test_split_percentage(session: Session) -> None:
+    """Test Pane.split() with percentage parameter."""
+    window = session.new_window(window_name="test_split_pct")
+    window.resize(height=40, width=80)
+    pane = window.active_pane
+    assert pane is not None
+
+    new_pane = pane.split(percentage=25)
+    assert new_pane in window.panes
+    assert len(window.panes) == 2
+
+    # The new pane should be roughly 25% of the window height
+    new_pane.refresh()
+    assert new_pane.pane_height is not None
+    assert int(new_pane.pane_height) <= 15  # ~25% of 40
+
+
+def test_split_percentage_size_mutual_exclusion(session: Session) -> None:
+    """Test that size and percentage are mutually exclusive."""
+    window = session.new_window(window_name="test_split_mutex")
+    pane = window.active_pane
+    assert pane is not None
+    with pytest.raises(ValueError, match="Cannot specify both"):
+        pane.split(size=10, percentage=50)