Avoid creating occlusions for editor blocks, since these block mouse wheel events (#20649)

Just block mouse down events, and in the case of the inline assist
prompt, set the default cursor.

Release Notes:

- N/A

Co-authored-by: Richard <richard@zed.dev>

crates/diagnostics/src/diagnostics.rs

@@ -795,7 +795,7 @@ fn diagnostic_header_renderer(diagnostic: Diagnostic) -> RenderBlock {
         let highlight_style: HighlightStyle = cx.theme().colors().text_accent.into();
         h_flex()
             .id(DIAGNOSTIC_HEADER)
-            .occlude()
+            .block_mouse_down()
             .h(2. * cx.line_height())
             .pl_10()
             .pr_5()