sl0p: stop hide_default_pane_names blanking the plugin API

`hide_default_pane_names` is a frame-rendering option, but it was applied
inside current_title() -- which is also what fills PaneInfo.title. Every plugin
that lists panes therefore received an empty string for any pane whose program
had never set a title.

For the bundled session manager that is untidy. For a fuzzy pane finder it is
broken: the rows are identical AND unsearchable, so the plugin's whole purpose
stops working.

The blanking moves to the frame, which already had a blank_title concept, and
current_title() goes back to reporting a real title to everything that asks.
Frames still show nothing for an unnamed pane, named panes still show their
name, and PaneInfo reports "Pane #N" again.

Changes a value rather than adding a field, so it needs no protobuf
regeneration.

Only observable without a shell that sets a title: verified by disabling the
zsh hook, where the pane finder previously listed three identical blank rows
and now lists Pane #1/#2/#3. That masking is also why this went unnoticed --
and why I flagged it twice without confirming it was actually reachable.

3 files changed, 91 insertions, 21 deletions

diff --git a/README.md b/README.md
index 3d8d73d6..56d1355b 100644
--- a/README.md
+++ b/README.md
@@ -147,6 +147,11 @@ Two deliberate behaviours:
   It protected little in any case: expanding applies `BASE` or a grid, so a
   manual arrangement is replaced by a collapse/expand round trip either way.
 
+Note this affects **frame rendering only**. Panes still report their `Pane #N`
+placeholder to the plugin API, so anything that lists panes — the session
+manager, a pane finder — can still label and search them. Blanking it there too
+left such plugins with rows they could neither identify nor fuzzy-match.
+
 ### Capping the stacked-pane picker
 
 A stack reserves one row per member plus a padding row, so a deep stack spends
diff --git a/zellij-server/src/panes/terminal_pane.rs b/zellij-server/src/panes/terminal_pane.rs
index 735bdc9d..8085e6be 100644
--- a/zellij-server/src/panes/terminal_pane.rs
+++ b/zellij-server/src/panes/terminal_pane.rs
@@ -583,7 +583,11 @@ impl Pane for TerminalPane {
         } else {
             self.current_title()
         };
-        let pane_title = if frame_params.blank_title {
+        let pane_title = if frame_params.blank_title
+            || (self.has_placeholder_title() && self.style.hide_default_pane_names)
+        {
+            // nothing named this pane: draw no title rather than "Pane #3".
+            // Read from style at render time so the option hot-reloads.
             String::new()
         } else if let Some(text_color_override) = self
             .pane_frame_color_override
@@ -1219,30 +1223,27 @@ impl Pane for TerminalPane {
     fn current_title(&self) -> String {
         // precedence: user-given name > title set by the running program >
         // the "Pane #N" placeholder
+        //
+        // Deliberately does NOT consult `hide_default_pane_names`. That is a
+        // frame-rendering option, and blanking here leaked it into everything
+        // that asks a pane for its title -- including PaneInfo, so every plugin
+        // listing panes (the pane finder, the session manager) got empty rows
+        // it could neither label nor search. The frame does its own blanking;
+        // see `has_placeholder_title`.
         if !self.pane_name.is_empty() {
             return self.pane_name.to_owned();
         }
         if let Some(program_title) = self.grid.title.as_deref() {
             return program_title.into();
         }
-        if self.pane_title_is_placeholder && self.style.hide_default_pane_names {
-            // nothing has named this pane: show no title at all rather than
-            // "Pane #3". Read from style at render time so the option
-            // hot-reloads and the placeholder is still there if it is turned
-            // back off.
-            return String::new();
-        }
         self.pane_title.clone()
     }
     fn stack_list_entry_label(&self) -> String {
-        // `hide_default_pane_names` deliberately does NOT apply here.
-        //
-        // In a frame, an empty title is fine: the pane is still visible, sized
-        // and positioned, so you can tell what it is by looking at it. In the
-        // stack list the label is the ENTIRE affordance -- blanking it leaves a
-        // row that cannot be identified or distinguished from its neighbours.
-        // So the stack list falls back to the "Pane #N" placeholder even when
-        // frames are configured to hide it.
+        // `hide_default_pane_names` does not reach current_title() any more, so
+        // this gets the "Pane #N" placeholder rather than an empty string. Kept
+        // as a floor anyway: in a frame an empty title is merely clean, but in
+        // the stack list the label is the ENTIRE affordance, and a blank row
+        // cannot be identified or told apart from its neighbours.
         let title = match self.current_title() {
             title if title.is_empty() => self.pane_title.clone(),
             title => title,
@@ -1451,6 +1452,12 @@ impl Pane for TerminalPane {
 }
 
 impl TerminalPane {
+    /// True while nothing has named this pane, so its title is still the
+    /// auto-generated "Pane #N". Inherent rather than part of the Pane trait:
+    /// only terminal panes have a placeholder title to hide.
+    fn has_placeholder_title(&self) -> bool {
+        self.pane_name.is_empty() && self.grid.title.is_none() && self.pane_title_is_placeholder
+    }
     fn title_with_bell_indicator(&self, title: String) -> String {
         if self.has_bell_notification {
             format!("{} [!]", title)
diff --git a/zellij-server/src/tab/unit/tab_integration_tests.rs b/zellij-server/src/tab/unit/tab_integration_tests.rs
index d59e86b6..eb1df250 100644
--- a/zellij-server/src/tab/unit/tab_integration_tests.rs
+++ b/zellij-server/src/tab/unit/tab_integration_tests.rs
@@ -15972,13 +15972,13 @@ fn sl0p_stack_list_entry_keeps_a_label_when_default_pane_names_are_hidden() {
     );
 
     for (pane_id, pane) in tab.tiled_panes.panes.iter() {
-        // precondition: the option really is blanking the frame title, or this
-        // test would pass for the wrong reason
+        // The option no longer reaches current_title(): it is applied by the
+        // frame at render time, so everything that ASKS a pane for its title
+        // still gets a real one. See the PaneInfo test below.
         assert_eq!(
             pane.current_title(),
-            "",
-            "precondition: {:?} should have a blank FRAME title while \
-             hide_default_pane_names is on",
+            format!("Pane #{}", pane_id_index(pane_id)),
+            "{:?} should still report its placeholder title",
             pane_id
         );
         let label = pane.stack_list_entry_label();
@@ -16746,3 +16746,61 @@ fn sl0p_stack_list_style_changes_the_cell_width_the_layout_reserves() {
         minimal.cells.len()
     );
 }
+
+#[cfg(test)]
+fn pane_id_index(pane_id: &PaneId) -> u32 {
+    match pane_id {
+        PaneId::Terminal(id) => *id,
+        PaneId::Plugin(id) => *id,
+    }
+}
+
+#[test]
+fn sl0p_hiding_default_pane_names_does_not_blank_the_plugin_api() {
+    // REGRESSION: `hide_default_pane_names` is a frame-rendering option, but it
+    // used to be applied inside current_title() -- which is also what fills
+    // PaneInfo.title. Every plugin listing panes (the pane finder, the session
+    // manager) therefore got empty rows it could neither label nor fuzzy-match,
+    // for any pane whose program never set a title.
+    let size = Size {
+        cols: 121,
+        rows: 20,
+    };
+    let client_id = 1;
+    let mut tab = create_new_tab(size, ModeInfo::default());
+    tab.new_pane(
+        PaneId::Terminal(2),
+        None,
+        None,
+        false,
+        true,
+        NewPanePlacement::default(),
+        Some(client_id),
+        None,
+    )
+    .unwrap();
+    tab.update_frame_layout(
+        zellij_utils::data::PaneTitleAlignment::Left,
+        0,
+        0,
+        zellij_utils::data::PanePadding::default(),
+        0,
+        true, // hide_default_pane_names
+    );
+
+    let current_pane_group = HashMap::new();
+    for (pane_id, pane) in tab.tiled_panes.panes.iter() {
+        let pane_info = super::pane_info_for_pane(pane_id, pane, &current_pane_group);
+        assert!(
+            !pane_info.title.trim().is_empty(),
+            "{:?} reported an empty title to the plugin API, so a pane list \
+             cannot label or search it",
+            pane_id
+        );
+        assert_eq!(
+            pane_info.title,
+            format!("Pane #{}", pane_id_index(pane_id)),
+            "the API should report the real placeholder title"
+        );
+    }
+}