Show diagnostics on hover (#4192)

Fixes https://github.com/zed-industries/community/issues/2426

Release Notes:

- Fixed a bug that was preventing diagnostics from being shown on hover.

crates/editor/src/hover_popover.rs

@@ -545,6 +545,7 @@ impl DiagnosticPopover {
 
         div()
             .id("diagnostic")
+            .elevation_2(cx)
             .overflow_y_scroll()
             .px_2()
             .py_1()