Fix scrollbar not always being on top (#20665)

Set the elevation of the scrollbar to 1 borderless, so that the blue
outline is no longer above the scrollbar.

Closes #19875

Release Notes:

- N/A

---------

Co-authored-by: Peter Tripp <peter@zed.dev>

crates/ui/src/components/scrollbar.rs

@@ -228,7 +228,9 @@ impl Element for Scrollbar {
     ) {
         cx.with_content_mask(Some(ContentMask { bounds }), |cx| {
             let colors = cx.theme().colors();
-            let thumb_background = colors.scrollbar_thumb_background;
+            let thumb_background = colors
+                .surface_background
+                .blend(colors.scrollbar_thumb_background);
             let is_vertical = self.kind == ScrollbarAxis::Vertical;
             let extra_padding = px(5.0);
             let padded_bounds = if is_vertical {