feat: make ui.pane_borders a three-state auto/always/off mode (#3234)

* feat: make ui.pane_borders a three-state auto/always/off mode

Legacy booleans keep parsing (true = auto, false = off); "always" frames
a lone pane while pane_outer_borders is enabled.

* fix: align pane border reload test and documentation

---------

Co-authored-by: Rudolf Schmidt <me@rudionrails.com>
Co-authored-by: Jonathan Liebig <jonathan.liebig@gmail.com>

1 files changed, 89 insertions, 6 deletions

diff --git a/src/config/model.rs b/src/config/model.rs
index a400382c..604fe9ad 100644
--- a/src/config/model.rs
+++ b/src/config/model.rs
@@ -844,6 +844,60 @@ pub enum TabBarPositionConfig {
     Bottom,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum PaneBordersConfig {
+    #[default]
+    Auto,
+    Always,
+    Off,
+}
+
+impl PaneBordersConfig {
+    pub fn draws_borders(self) -> bool {
+        !matches!(self, Self::Off)
+    }
+
+    pub fn shows_borders(self, multi_pane: bool) -> bool {
+        self.draws_borders() && (multi_pane || matches!(self, Self::Always))
+    }
+}
+
+impl<'de> Deserialize<'de> for PaneBordersConfig {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        struct PaneBordersVisitor;
+
+        impl<'de> de::Visitor<'de> for PaneBordersVisitor {
+            type Value = PaneBordersConfig;
+
+            fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+                formatter.write_str("\"auto\", \"always\", \"off\", or a legacy boolean")
+            }
+
+            fn visit_bool<E: de::Error>(self, value: bool) -> Result<Self::Value, E> {
+                Ok(if value {
+                    PaneBordersConfig::Auto
+                } else {
+                    PaneBordersConfig::Off
+                })
+            }
+
+            fn visit_str<E: de::Error>(self, value: &str) -> Result<Self::Value, E> {
+                match value {
+                    "auto" => Ok(PaneBordersConfig::Auto),
+                    "always" => Ok(PaneBordersConfig::Always),
+                    "off" => Ok(PaneBordersConfig::Off),
+                    other => Err(E::invalid_value(de::Unexpected::Str(other), &self)),
+                }
+            }
+        }
+
+        deserializer.deserialize_any(PaneBordersVisitor)
+    }
+}
+
 #[derive(Debug, Deserialize)]
 #[serde(default)]
 pub struct UiConfig {
@@ -876,8 +930,12 @@ pub struct UiConfig {
     pub prompt_new_tab_name: bool,
     /// Ask for a workspace name before interactive creation. Default: false.
     pub prompt_new_workspace_name: bool,
-    /// Draw borders around split panes. Default: true.
-    pub pane_borders: bool,
+    /// Draw borders around split panes. auto draws them only for split panes,
+    /// always also frames a lone pane (only while pane_outer_borders is
+    /// enabled, since every edge of a lone pane is an outer edge), off
+    /// disables them. Legacy booleans map true to auto and false to off.
+    /// Default: auto.
+    pub pane_borders: PaneBordersConfig,
     /// Draw borders along the outside edge of the pane area. Default: true.
     pub pane_outer_borders: bool,
     /// Draw interactive scrollbars beside terminal panes. Default: true.
@@ -1115,7 +1173,7 @@ impl Default for UiConfig {
             confirm_close: true,
             prompt_new_tab_name: true,
             prompt_new_workspace_name: false,
-            pane_borders: true,
+            pane_borders: PaneBordersConfig::Auto,
             pane_outer_borders: true,
             pane_scrollbars: true,
             pane_gaps: true,
@@ -1390,9 +1448,34 @@ status_indicators = "symbols"
     }
 
     #[test]
+    fn pane_borders_legacy_booleans_map_to_modes() {
+        let enabled: Config = toml::from_str("[ui]\npane_borders = true").unwrap();
+        assert_eq!(enabled.ui.pane_borders, PaneBordersConfig::Auto);
+
+        let disabled: Config = toml::from_str("[ui]\npane_borders = false").unwrap();
+        assert_eq!(disabled.ui.pane_borders, PaneBordersConfig::Off);
+
+        let auto: Config = toml::from_str("[ui]\npane_borders = \"auto\"").unwrap();
+        assert_eq!(auto.ui.pane_borders, PaneBordersConfig::Auto);
+
+        let off: Config = toml::from_str("[ui]\npane_borders = \"off\"").unwrap();
+        assert_eq!(off.ui.pane_borders, PaneBordersConfig::Off);
+
+        let unknown = toml::from_str::<Config>("[ui]\npane_borders = \"framed\"")
+            .unwrap_err()
+            .to_string();
+        assert!(unknown.contains("\"auto\", \"always\", \"off\", or a legacy boolean"));
+
+        let wrong_type = toml::from_str::<Config>("[ui]\npane_borders = 3")
+            .unwrap_err()
+            .to_string();
+        assert!(wrong_type.contains("\"auto\", \"always\", \"off\", or a legacy boolean"));
+    }
+
+    #[test]
     fn pane_appearance_defaults_and_parse() {
         let default_config = Config::default();
-        assert!(default_config.ui.pane_borders);
+        assert_eq!(default_config.ui.pane_borders, PaneBordersConfig::Auto);
         assert!(default_config.ui.pane_outer_borders);
         assert!(default_config.ui.pane_scrollbars);
         assert!(default_config.ui.pane_gaps);
@@ -1407,7 +1490,7 @@ status_indicators = "symbols"
 
         let toml = r#"
 [ui]
-pane_borders = false
+pane_borders = "always"
 pane_outer_borders = false
 pane_scrollbars = false
 pane_gaps = true
@@ -1424,7 +1507,7 @@ tab_bar_right = [
 tab_bar_right_separator = " · "
 "#;
         let config: Config = toml::from_str(toml).unwrap();
-        assert!(!config.ui.pane_borders);
+        assert_eq!(config.ui.pane_borders, PaneBordersConfig::Always);
         assert!(!config.ui.pane_outer_borders);
         assert!(!config.ui.pane_scrollbars);
         assert!(config.ui.pane_gaps);