Avoid excessive diagnostics refreshes (#21173)

Attempts to reduce the diagnostics flicker, when editing very
fundamental parts of the large code base in Rust.


https://github.com/user-attachments/assets/dc3f9c21-8c6e-48db-967b-040649fd00da

Release Notes:

- N/A

crates/diagnostics/src/diagnostics.rs

@@ -33,6 +33,7 @@ use std::{
     mem,
     ops::Range,
     sync::Arc,
+    time::Duration,
 };
 use theme::ActiveTheme;
 pub use toolbar_controls::ToolbarControls;
@@ -82,6 +83,8 @@ struct DiagnosticGroupState {
 
 impl EventEmitter<EditorEvent> for ProjectDiagnosticsEditor {}
 
+const DIAGNOSTICS_UPDATE_DEBOUNCE: Duration = Duration::from_millis(50);
+
 impl Render for ProjectDiagnosticsEditor {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
         let child = if self.path_states.is_empty() {
@@ -198,6 +201,9 @@ impl ProjectDiagnosticsEditor {
         }
         let project_handle = self.project.clone();
         self.update_excerpts_task = Some(cx.spawn(|this, mut cx| async move {
+            cx.background_executor()
+                .timer(DIAGNOSTICS_UPDATE_DEBOUNCE)
+                .await;
             loop {
                 let Some((path, language_server_id)) = this.update(&mut cx, |this, _| {
                     let Some((path, language_server_id)) = this.paths_to_update.pop_first() else {