sl0p: optionally hide the 'Pane #N' placeholder title

`ui { pane_frames { hide_default_pane_names true } }`, default false.

A pane now shows no title at all until something actually names it: the user
renaming it (including by double-clicking the title row) or the running
program setting a title. Upstream always shows an auto-generated 'Pane #N'.

Implemented by tracking whether pane_title is still the constructor's
placeholder, NOT by matching the string: a program that legitimately titles
itself 'Pane #3' must not be mistaken for an unnamed pane. set_title clears
the flag, since something then chose that title.

current_title reads the option at render time rather than baking an empty
title in at construction, so it hot-reloads and turning the option back off
restores the placeholder on existing panes.

Display precedence is unchanged and now explicit in the code:
    user-given name > title set by the running program > placeholder

render_terminal_title already fell back to an empty string rather than the
placeholder, so the terminal WINDOW title is unaffected.

Empty titles were already handled by the frame renderer
(render_title_left_side returns None for an empty title), including the
sl0p centred/right alignment path.

Baselines held: zellij-server 1436/30, zellij-utils 436/5.

6 files changed, 20 insertions, 0 deletions

diff --git a/zellij-utils/src/data.rs b/zellij-utils/src/data.rs
index 9f9205db..2caedf8f 100644
--- a/zellij-utils/src/data.rs
+++ b/zellij-utils/src/data.rs
@@ -1468,6 +1468,8 @@ pub struct Style {
     /// Desaturation strength (0-100) applied to the CONTENT of panes this
     /// client is not focused on. 0 = off.
     pub inactive_pane_grayscale: u8,
+    /// Suppress the auto-generated "Pane #N" placeholder title.
+    pub hide_default_pane_names: bool,
 }
 
 #[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 77260ff8..7b870332 100644
--- a/zellij-utils/src/input/config.rs
+++ b/zellij-utils/src/input/config.rs
@@ -1457,6 +1457,7 @@ mod config_test {
                 padding: Default::default(),
                 gap_aspect: 2, // parser default; see FrameConfig::default()
                 inactive_grayscale: 0,
+                hide_default_pane_names: false,
             },
         };
         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 e1371f85..45ae763b 100644
--- a/zellij-utils/src/input/theme.rs
+++ b/zellij-utils/src/input/theme.rs
@@ -40,6 +40,9 @@ pub struct FrameConfig {
     /// How strongly to desaturate the CONTENT of unfocused panes, 0-100.
     /// 0 disables it entirely (the default).
     pub inactive_grayscale: u8,
+    /// Omit the auto-generated "Pane #N" placeholder title, so a pane shows no
+    /// name until the user sets one or the running program sets a title.
+    pub hide_default_pane_names: bool,
 }
 
 /// Hand-written rather than derived: `gap_aspect` must default to 2 to match
@@ -56,6 +59,7 @@ impl Default for FrameConfig {
             padding: Default::default(),
             gap_aspect: 2,
             inactive_grayscale: 0,
+            hide_default_pane_names: false,
         }
     }
 }
@@ -83,6 +87,7 @@ impl FrameConfig {
         merged.padding = other.padding;
         merged.gap_aspect = other.gap_aspect;
         merged.inactive_grayscale = other.inactive_grayscale;
+        merged.hide_default_pane_names = other.hide_default_pane_names;
         merged
     }
 }
diff --git a/zellij-utils/src/kdl/mod.rs b/zellij-utils/src/kdl/mod.rs
index cae59ff6..8d7f4d9d 100644
--- a/zellij-utils/src/kdl/mod.rs
+++ b/zellij-utils/src/kdl/mod.rs
@@ -5599,6 +5599,9 @@ impl UiConfig {
                 kdl_children_property_first_arg_as_i64!(pane_frames, "inactive_grayscale")
                     .unwrap_or(0)
                     .clamp(0, 100) as u8;
+            let hide_default_pane_names =
+                kdl_children_property_first_arg_as_bool!(pane_frames, "hide_default_pane_names")
+                    .unwrap_or(false);
             let frame_config = FrameConfig {
                 rounded_corners,
                 hide_session_name,
@@ -5607,6 +5610,7 @@ impl UiConfig {
                 padding,
                 gap_aspect,
                 inactive_grayscale,
+                hide_default_pane_names,
             };
             ui_config.pane_frames = frame_config;
         }
@@ -5664,6 +5668,12 @@ impl UiConfig {
             }
             frame_config_children.nodes_mut().push(padding);
         }
+        if self.pane_frames.hide_default_pane_names {
+            has_ui_config = true;
+            let mut node = KdlNode::new("hide_default_pane_names");
+            node.push(KdlValue::Bool(true));
+            frame_config_children.nodes_mut().push(node);
+        }
         if self.pane_frames.inactive_grayscale > 0 {
             has_ui_config = true;
             let mut node = KdlNode::new("inactive_grayscale");
diff --git a/zellij-utils/src/plugin_api/event.rs b/zellij-utils/src/plugin_api/event.rs
index 0303ec1f..0dafec64 100644
--- a/zellij-utils/src/plugin_api/event.rs
+++ b/zellij-utils/src/plugin_api/event.rs
@@ -2497,6 +2497,7 @@ fn serialize_mode_update_event_with_non_default_values() {
             pane_gap_y: Default::default(),
             pane_padding: Default::default(),
             inactive_pane_grayscale: Default::default(),
+            hide_default_pane_names: 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 cb896c76..cf554351 100644
--- a/zellij-utils/src/plugin_api/style.rs
+++ b/zellij-utils/src/plugin_api/style.rs
@@ -30,6 +30,7 @@ impl TryFrom<ProtobufStyle> for Style {
             pane_gap_y: Default::default(),
             pane_padding: Default::default(),
             inactive_pane_grayscale: Default::default(),
+            hide_default_pane_names: Default::default(),
         })
     }
 }