Ensure horizontal scrollbars show as needed (#30964)

This PR fixes an issue where the horizontal scrollbar was sometimes not
rendered despite being needed for the outline and project panels.

The issue occured since `self.width` does not neccessarily have to be
set when the scrollbar is rendered (it is only set on panel resize).
However, the check for a `width` is not needed at all since the
scrollbar constructor determines whether a scrollbar has to be rendered
or not. Hence, this does not need to be special-cased.

Furthermore, since `Scrollbar::horizontal()` returns `Some(...)` when a
scrollbar needs to be rendered, we do not have to check for this
seperately on the scroll handle and can just map on the option. This
simplifies the code a bit.

| `main` | This PR |
| --- | --- | 
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![main](https://github.com/user-attachments/assets/db1d4524-716e-42c1-a6f9-7cfd59c94b30)
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![PR](https://github.com/user-attachments/assets/12536d28-616e-487d-b948-653f53da36b4)
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Release Notes:

- Fixed an issue where the horizontal scrollbar would not render in the
project and outline panels.

crates/outline_panel/src/outline_panel.rs

@@ -4335,19 +4335,7 @@ impl OutlinePanel {
         {
             return None;
         }
-
+        Scrollbar::horizontal(self.horizontal_scrollbar_state.clone()).map(|scrollbar| {
-        let scroll_handle = self.scroll_handle.0.borrow();
-        let longest_item_width = scroll_handle
-            .last_item_size
-            .filter(|size| size.contents.width > size.item.width)?
-            .contents
-            .width
-            .0 as f64;
-        if longest_item_width < scroll_handle.base_handle.bounds().size.width.0 as f64 {
-            return None;
-        }
-
-        Some(
             div()
                 .occlude()
                 .id("project-panel-horizontal-scroll")
@@ -4384,12 +4372,8 @@ impl OutlinePanel {
                 .bottom_0()
                 .h(px(12.))
                 .cursor_default()
-                .when(self.width.is_some(), |this| {
+                .child(scrollbar)
-                    this.children(Scrollbar::horizontal(
+        })
-                        self.horizontal_scrollbar_state.clone(),
-                    ))
-                }),
-        )
     }
 
     fn should_show_scrollbar(cx: &App) -> bool {

crates/project_panel/src/project_panel.rs

@@ -4340,19 +4340,7 @@ impl ProjectPanel {
         {
             return None;
         }
-
+        Scrollbar::horizontal(self.horizontal_scrollbar_state.clone()).map(|scrollbar| {
-        let scroll_handle = self.scroll_handle.0.borrow();
-        let longest_item_width = scroll_handle
-            .last_item_size
-            .filter(|size| size.contents.width > size.item.width)?
-            .contents
-            .width
-            .0 as f64;
-        if longest_item_width < scroll_handle.base_handle.bounds().size.width.0 as f64 {
-            return None;
-        }
-
-        Some(
             div()
                 .occlude()
                 .id("project-panel-horizontal-scroll")
@@ -4389,12 +4377,8 @@ impl ProjectPanel {
                 .bottom_1()
                 .h(px(12.))
                 .cursor_default()
-                .when(self.width.is_some(), |this| {
+                .child(scrollbar)
-                    this.children(Scrollbar::horizontal(
+        })
-                        self.horizontal_scrollbar_state.clone(),
-                    ))
-                }),
-        )
     }
 
     fn dispatch_context(&self, window: &Window, cx: &Context<Self>) -> KeyContext {