sl0p: configurable pane title alignment

Adds `ui { pane_frames { title_alignment "left"|"center"|"right" } }`.

Full-frame panes previously hardcoded the title flush left, while frameless
panes (pane_frame_style "titles") already centered it via
compose_bracketed_title. This brings full frames up to parity and makes the
position configurable.

Implementation notes:
- The value rides on `Style`, which is already threaded to FrameParams, so no
  new plumbing through Tab/PaneContentsAndUi was needed.
- It is deliberately NOT round-tripped over the plugin protobuf boundary:
  plugins have no use for frame title alignment and adding a proto field
  would force regenerating the committed prost assets in assets/prost/.
- Left keeps the original code path untouched, so default configs render
  exactly as before.
- The geometry loop clamps the title start against the right-side indicators
  and breaks on `>=` rather than `==`, so a mis-clamped segment cannot spin.

Tests: alignment ordering, frame width preservation across widths, and
narrow-pane safety (cols 1..24 must not panic or corrupt width).

6 files changed, 68 insertions, 0 deletions

diff --git a/zellij-utils/src/data.rs b/zellij-utils/src/data.rs
index 12e66002..f4ce534f 100644
--- a/zellij-utils/src/data.rs
+++ b/zellij-utils/src/data.rs
@@ -1384,11 +1384,45 @@ pub struct Palette {
     pub brown: PaletteColor,
 }
 
+/// Where the pane title sits inside a pane's frame.
+///
+/// NOTE: this currently only affects panes drawn with FULL frames. In frameless
+/// (`pane_frame_style "titles"`) mode the title goes through
+/// `compose_bracketed_title`, which already centers it unconditionally.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Default, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum PaneTitleAlignment {
+    #[default]
+    Left,
+    Center,
+    Right,
+}
+
+impl std::str::FromStr for PaneTitleAlignment {
+    type Err = String;
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.trim().to_lowercase().as_str() {
+            "left" => Ok(PaneTitleAlignment::Left),
+            "center" | "centre" => Ok(PaneTitleAlignment::Center),
+            "right" => Ok(PaneTitleAlignment::Right),
+            e => Err(format!(
+                "Unknown pane title alignment: '{}' (expected 'left', 'center' or 'right')",
+                e
+            )),
+        }
+    }
+}
+
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, Default, Serialize, Deserialize)]
 pub struct Style {
     pub colors: Styling,
     pub rounded_corners: bool,
     pub hide_session_name: bool,
+    /// NOT round-tripped through the plugin protobuf API on purpose: plugins
+    /// have no use for it and adding a proto field would force regenerating
+    /// the committed prost assets. Defaults to Left when coming back from a
+    /// ProtobufStyle.
+    pub pane_title_alignment: PaneTitleAlignment,
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Deserialize, Serialize)]
diff --git a/zellij-utils/src/input/config.rs b/zellij-utils/src/input/config.rs
index 1b917864..9387aef3 100644
--- a/zellij-utils/src/input/config.rs
+++ b/zellij-utils/src/input/config.rs
@@ -1407,6 +1407,7 @@ mod config_test {
             pane_frames: FrameConfig {
                 rounded_corners: true,
                 hide_session_name: true,
+                title_alignment: Default::default(),
             },
         };
         assert_eq!(config.ui, expected_ui_config, "Ui config defined in config");
diff --git a/zellij-utils/src/input/theme.rs b/zellij-utils/src/input/theme.rs
index b4fbc630..0930bc15 100644
--- a/zellij-utils/src/input/theme.rs
+++ b/zellij-utils/src/input/theme.rs
@@ -26,6 +26,7 @@ impl UiConfig {
 pub struct FrameConfig {
     pub rounded_corners: bool,
     pub hide_session_name: bool,
+    pub title_alignment: crate::data::PaneTitleAlignment,
 }
 
 impl FrameConfig {
@@ -33,6 +34,7 @@ impl FrameConfig {
         let mut merged = self.clone();
         merged.rounded_corners = other.rounded_corners;
         merged.hide_session_name = other.hide_session_name;
+        merged.title_alignment = other.title_alignment;
         merged
     }
 }
diff --git a/zellij-utils/src/kdl/mod.rs b/zellij-utils/src/kdl/mod.rs
index 8e86445f..bc188aed 100644
--- a/zellij-utils/src/kdl/mod.rs
+++ b/zellij-utils/src/kdl/mod.rs
@@ -5524,9 +5524,21 @@ impl UiConfig {
                     .unwrap_or(false);
             let hide_session_name =
                 kdl_get_child_entry_bool_value!(pane_frames, "hide_session_name").unwrap_or(false);
+            let title_alignment = match kdl_children_property_first_arg_as_string!(
+                pane_frames,
+                "title_alignment"
+            ) {
+                Some(string) => {
+                    crate::data::PaneTitleAlignment::from_str(string).map_err(|e| {
+                        ConfigError::new_kdl_error(e, kdl_ui_config.span().offset(), kdl_ui_config.span().len())
+                    })?
+                },
+                None => crate::data::PaneTitleAlignment::default(),
+            };
             let frame_config = FrameConfig {
                 rounded_corners,
                 hide_session_name,
+                title_alignment,
             };
             ui_config.pane_frames = frame_config;
         }
@@ -5550,6 +5562,19 @@ impl UiConfig {
             hide_session_name.push(KdlValue::Bool(true));
             frame_config_children.nodes_mut().push(hide_session_name);
         }
+        if self.pane_frames.title_alignment != crate::data::PaneTitleAlignment::default() {
+            has_ui_config = true;
+            let mut title_alignment = KdlNode::new("title_alignment");
+            title_alignment.push(KdlValue::String(
+                match self.pane_frames.title_alignment {
+                    crate::data::PaneTitleAlignment::Left => "left",
+                    crate::data::PaneTitleAlignment::Center => "center",
+                    crate::data::PaneTitleAlignment::Right => "right",
+                }
+                .to_string(),
+            ));
+            frame_config_children.nodes_mut().push(title_alignment);
+        }
         if has_ui_config {
             frame_config.set_children(frame_config_children);
             ui_config_children.nodes_mut().push(frame_config);
diff --git a/zellij-utils/src/plugin_api/event.rs b/zellij-utils/src/plugin_api/event.rs
index 970f5e2c..0189d11b 100644
--- a/zellij-utils/src/plugin_api/event.rs
+++ b/zellij-utils/src/plugin_api/event.rs
@@ -2492,6 +2492,7 @@ fn serialize_mode_update_event_with_non_default_values() {
             // TODO: replace default
             rounded_corners: true,
             hide_session_name: false,
+            pane_title_alignment: Default::default(),
         },
         capabilities: PluginCapabilities { arrow_fonts: false },
         session_name: Some("my awesome test session".to_owned()),
diff --git a/zellij-utils/src/plugin_api/style.rs b/zellij-utils/src/plugin_api/style.rs
index eb9eebcf..2f3ff298 100644
--- a/zellij-utils/src/plugin_api/style.rs
+++ b/zellij-utils/src/plugin_api/style.rs
@@ -21,6 +21,11 @@ impl TryFrom<ProtobufStyle> for Style {
             colors: s,
             rounded_corners: protobuf_style.rounded_corners,
             hide_session_name: protobuf_style.hide_session_name,
+            // deliberately not carried over the plugin protobuf boundary; see
+            // the field docs on Style. Plugins never need the frame's title
+            // alignment, and adding a proto field would mean regenerating the
+            // committed prost assets.
+            pane_title_alignment: Default::default(),
         })
     }
 }