Add scrollbars to outline panel (#19969)

Part of https://github.com/zed-industries/zed/issues/15324


![image](https://github.com/user-attachments/assets/4f32d585-9bd2-46be-8234-3658a71906ee)

Repeats the approach used in the project panel.

Release Notes:

- Added scrollbars to outline panel

---------

Co-authored-by: Nate Butler <nate@zed.dev>

assets/settings/default.json

@@ -414,6 +414,23 @@
       // 2. Never show indent guides:
       //    "never"
       "show": "always"
+    },
+    /// Scrollbar-related settings
+    "scrollbar": {
+      /// When to show the scrollbar in the project panel.
+      /// This setting can take four values:
+      ///
+      /// 1. null (default): Inherit editor settings
+      /// 2. Show the scrollbar if there's important information or
+      ///    follow the system's configured behavior (default):
+      ///   "auto"
+      /// 3. Match the system's configured behavior:
+      ///    "system"
+      /// 4. Always show the scrollbar:
+      ///    "always"
+      /// 5. Never show the scrollbar:
+      ///    "never"
+      "show": null
     }
   },
   "collaboration_panel": {

crates/outline_panel/src/outline_panel_settings.rs

@@ -1,3 +1,4 @@
+use editor::ShowScrollbar;
 use gpui::Pixels;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
@@ -29,6 +30,23 @@ pub struct OutlinePanelSettings {
     pub indent_guides: IndentGuidesSettings,
     pub auto_reveal_entries: bool,
     pub auto_fold_dirs: bool,
+    pub scrollbar: ScrollbarSettings,
+}
+
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+pub struct ScrollbarSettings {
+    /// When to show the scrollbar in the project panel.
+    ///
+    /// Default: inherits editor scrollbar settings
+    pub show: Option<ShowScrollbar>,
+}
+
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+pub struct ScrollbarSettingsContent {
+    /// When to show the scrollbar in the project panel.
+    ///
+    /// Default: inherits editor scrollbar settings
+    pub show: Option<Option<ShowScrollbar>>,
 }
 
 #[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
@@ -85,6 +103,8 @@ pub struct OutlinePanelSettingsContent {
     pub auto_fold_dirs: Option<bool>,
     /// Settings related to indent guides in the outline panel.
     pub indent_guides: Option<IndentGuidesSettingsContent>,
+    /// Scrollbar-related settings
+    pub scrollbar: Option<ScrollbarSettingsContent>,
 }
 
 impl Settings for OutlinePanelSettings {

docs/src/configuring-zed.md

@@ -2271,6 +2271,9 @@ Run the `theme selector: toggle` action in the command palette to see a current
   "auto_fold_dirs": true,
   "indent_guides": {
     "show": "always"
+  },
+  "scrollbar": {
+    "show": null
   }
 }
 ```