project panel: Add a shadow in the last sticky item (#34042)

Follow-up to https://github.com/zed-industries/zed/pull/33994. This PR
adds a subtle shadow—built from an absolute-positioned div, due to
layering of items—to the last sticky item in the project panel when that
setting is turned on. This helps understand the block of items that is
currently sticky.

<img
src="https://github.com/user-attachments/assets/0e030e93-9bc6-42ff-8d0d-3e46f1986152"
width="300"/>

Would love to add indent guides to the items that are sticky as a next
step.

Release Notes:

- project panel: When `sticky_scroll` is true, the last item will now
have a subtle shadow to help visualizing the block of items that are
currently sticky.

crates/project_panel/src/project_panel.rs

@@ -23,7 +23,8 @@ use gpui::{
     ListSizingBehavior, Modifiers, ModifiersChangedEvent, MouseButton, MouseDownEvent,
     ParentElement, Pixels, Point, PromptLevel, Render, ScrollStrategy, Stateful, Styled,
     Subscription, Task, UniformListScrollHandle, WeakEntity, Window, actions, anchored, deferred,
-    div, point, px, size, transparent_white, uniform_list,
+    div, hsla, linear_color_stop, linear_gradient, point, px, size, transparent_white,
+    uniform_list,
 };
 use indexmap::IndexMap;
 use language::DiagnosticSeverity;
@@ -185,6 +186,7 @@ struct EntryDetails {
 #[derive(Debug, PartialEq, Eq, Clone)]
 struct StickyDetails {
     sticky_index: usize,
+    is_last: bool,
 }
 
 /// Permanently deletes the selected file or directory.
@@ -3928,8 +3930,24 @@ impl ProjectPanel {
             }
         };
 
+        let last_sticky_item = details.sticky.as_ref().map_or(false, |item| item.is_last);
+        let shadow_color_top = hsla(0.0, 0.0, 0.0, 0.15);
+        let shadow_color_bottom = hsla(0.0, 0.0, 0.0, 0.);
+        let sticky_shadow = div()
+            .absolute()
+            .left_0()
+            .bottom_neg_1p5()
+            .h_1p5()
+            .w_full()
+            .bg(linear_gradient(
+                0.,
+                linear_color_stop(shadow_color_top, 1.),
+                linear_color_stop(shadow_color_bottom, 0.),
+            ));
+
         div()
             .id(entry_id.to_proto() as usize)
+            .relative()
             .group(GROUP_NAME)
             .cursor_pointer()
             .rounded_none()
@@ -3938,6 +3956,7 @@ impl ProjectPanel {
             .border_r_2()
             .border_color(border_color)
             .hover(|style| style.bg(bg_hover_color).border_color(border_hover_color))
+            .when(is_sticky && last_sticky_item, |this| this.child(sticky_shadow))
             .when(!is_sticky, |this| {
                 this
                 .when(is_highlighted && folded_directory_drag_target.is_none(), |this| this.border_color(transparent_white()).bg(item_colors.drag_over))
@@ -4931,6 +4950,7 @@ impl ProjectPanel {
                     .unwrap_or_default();
                 let sticky_details = Some(StickyDetails {
                     sticky_index: index,
+                    is_last: index == sticky_parents.len() - 1,
                 });
                 let details = self.details_for_entry(
                     entry,