editor: Implement hover color for scrollbars (#28064)

This PR adds hover colors to the editor scrollbars:


https://github.com/user-attachments/assets/6600810e-7e8e-4dee-9bef-b7be303b5fe0

The color used here is the existing `scrollbar_thumb_hover_background`
color provided by themes.

Looking forward to feedback 😄 

Release Notes:

- Added hover state to editor scrollbars.

crates/editor/src/scroll.rs

@@ -123,6 +123,29 @@ impl OngoingScroll {
     }
 }
 
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum ScrollbarThumbState {
+    Idle,
+    Hovered,
+    Dragging,
+}
+
+#[derive(PartialEq, Eq)]
+pub struct ActiveScrollbarState {
+    axis: Axis,
+    thumb_state: ScrollbarThumbState,
+}
+
+impl ActiveScrollbarState {
+    pub fn new(axis: Axis, thumb_state: ScrollbarThumbState) -> Self {
+        ActiveScrollbarState { axis, thumb_state }
+    }
+
+    pub fn thumb_state_for_axis(&self, axis: Axis) -> Option<ScrollbarThumbState> {
+        (self.axis == axis).then_some(self.thumb_state)
+    }
+}
+
 pub struct ScrollManager {
     pub(crate) vertical_scroll_margin: f32,
     anchor: ScrollAnchor,
@@ -131,7 +154,7 @@ pub struct ScrollManager {
     last_autoscroll: Option<(gpui::Point<f32>, f32, f32, AutoscrollStrategy)>,
     show_scrollbars: bool,
     hide_scrollbar_task: Option<Task<()>>,
-    dragging_scrollbar: Option<Axis>,
+    active_scrollbar: Option<ActiveScrollbarState>,
     visible_line_count: Option<f32>,
     forbid_vertical_scroll: bool,
 }
@@ -145,7 +168,7 @@ impl ScrollManager {
             autoscroll_request: None,
             show_scrollbars: true,
             hide_scrollbar_task: None,
-            dragging_scrollbar: None,
+            active_scrollbar: None,
             last_autoscroll: None,
             visible_line_count: None,
             forbid_vertical_scroll: false,
@@ -322,24 +345,53 @@ impl ScrollManager {
         self.autoscroll_request.map(|(autoscroll, _)| autoscroll)
     }
 
+    pub fn active_scrollbar_state(&self) -> Option<&ActiveScrollbarState> {
+        self.active_scrollbar.as_ref()
+    }
+
     pub fn dragging_scrollbar_axis(&self) -> Option<Axis> {
-        self.dragging_scrollbar
+        self.active_scrollbar
+            .as_ref()
+            .map(|scrollbar| scrollbar.axis)
     }
 
     pub fn any_scrollbar_dragged(&self) -> bool {
-        self.dragging_scrollbar.is_some()
+        self.active_scrollbar
+            .as_ref()
+            .is_some_and(|scrollbar| scrollbar.thumb_state == ScrollbarThumbState::Dragging)
     }
 
-    pub fn set_dragged_scrollbar_axis(&mut self, axis: Axis, cx: &mut Context<Editor>) {
-        if self.dragging_scrollbar != Some(axis) {
-            self.dragging_scrollbar = Some(axis);
-            cx.notify();
-        }
+    pub fn set_hovered_scroll_thumb_axis(&mut self, axis: Axis, cx: &mut Context<Editor>) {
+        self.update_active_scrollbar_state(
+            Some(ActiveScrollbarState::new(
+                axis,
+                ScrollbarThumbState::Hovered,
+            )),
+            cx,
+        );
     }
 
-    pub fn reset_scrollbar_dragging_state(&mut self, cx: &mut Context<Editor>) {
-        if self.dragging_scrollbar.is_some() {
-            self.dragging_scrollbar = None;
+    pub fn set_dragged_scroll_thumb_axis(&mut self, axis: Axis, cx: &mut Context<Editor>) {
+        self.update_active_scrollbar_state(
+            Some(ActiveScrollbarState::new(
+                axis,
+                ScrollbarThumbState::Dragging,
+            )),
+            cx,
+        );
+    }
+
+    pub fn reset_scrollbar_state(&mut self, cx: &mut Context<Editor>) {
+        self.update_active_scrollbar_state(None, cx);
+    }
+
+    fn update_active_scrollbar_state(
+        &mut self,
+        new_state: Option<ActiveScrollbarState>,
+        cx: &mut Context<Editor>,
+    ) {
+        if self.active_scrollbar != new_state {
+            self.active_scrollbar = new_state;
             cx.notify();
         }
     }