Style collab panel (#3638)

This PR styles the collab panel.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

crates/ui2/src/components/disclosure.rs

@@ -1,6 +1,7 @@
-use crate::{prelude::*, Color, Icon, IconButton, IconSize};
 use gpui::ClickEvent;
 
+use crate::{prelude::*, Color, Icon, IconButton, IconSize};
+
 #[derive(IntoElement)]
 pub struct Disclosure {
     is_open: bool,

crates/ui2/src/components/list/list_item.rs

@@ -1,9 +1,10 @@
-use crate::{prelude::*, Disclosure};
 use gpui::{
     px, AnyElement, AnyView, ClickEvent, Div, MouseButton, MouseDownEvent, Pixels, Stateful,
 };
 use smallvec::SmallVec;
 
+use crate::{prelude::*, Disclosure};
+
 #[derive(IntoElement)]
 pub struct ListItem {
     id: ElementId,
@@ -192,10 +193,15 @@ impl RenderOnce for ListItem {
                             this.ml(self.indent_level as f32 * self.indent_step_size)
                         }
                     })
-                    .children(
-                        self.toggle
-                            .map(|is_open| Disclosure::new(is_open).on_toggle(self.on_toggle)),
-                    )
+                    .children(self.toggle.map(|is_open| {
+                        div()
+                            .flex()
+                            .absolute()
+                            .left(rems(-1.))
+                            .invisible()
+                            .group_hover("", |style| style.visible())
+                            .child(Disclosure::new(is_open).on_toggle(self.on_toggle))
+                    }))
                     .child(
                         h_stack()
                             .flex_1()