[outline] debounce updates

on relaod of the browser it happens that updates are triggered quite often, which causes cancellation of previous
requests. `DocumentSymbolProvider.provideDocumentSymbols` calls will throw on cancellation.

Signed-off-by: Alex Tugarev <alex.tugarev@typefox.io>

packages/monaco/src/browser/monaco-outline-contribution.ts

@@ -18,6 +18,8 @@ import { OutlineSymbolInformationNode } from '@theia/outline-view/lib/browser/ou
 import URI from "@theia/core/lib/common/uri";
 import { MonacoEditor } from './monaco-editor';
 
+import debounce = require('lodash.debounce');
+
 @injectable()
 export class MonacoOutlineContribution implements FrontendApplicationContribution {
 
@@ -35,15 +37,13 @@ export class MonacoOutlineContribution implements FrontendApplicationContributio
         this.outlineViewService.onDidChangeOpenState(async isOpen => {
             this.updateOutline();
         });
-        // let's skip the initial current Editor change event, as on reload it comes before the language sevrers have started,
+        // let's skip the initial current Editor change event, as on reload it comes before the language server have started,
         // resulting in an empty outline.
         setTimeout(() => {
-            this.editorManager.onCurrentEditorChanged(widget => this.updateOutlineForEditor(widget));
+            this.editorManager.onCurrentEditorChanged(debounce(widget => this.updateOutlineForEditor(widget), 50));
         }, 3000);
 
-        DocumentSymbolProviderRegistry.onDidChange(event => {
-            this.updateOutline();
-        });
+        DocumentSymbolProviderRegistry.onDidChange(debounce(event => this.updateOutline()));
 
         this.outlineViewService.onDidSelect(async node => {
             if (MonacoOutlineSymbolInformationNode.is(node) && node.parent) {
@@ -102,13 +102,18 @@ export class MonacoOutlineContribution implements FrontendApplicationContributio
         this.cancellationSource = new CancellationTokenSource();
         const token = this.cancellationSource.token;
         for (const documentSymbolProvider of documentSymbolProviders) {
-            const symbolInformation = await documentSymbolProvider.provideDocumentSymbols(model, token);
-            if (token.isCancellationRequested) {
+            try {
+                const symbolInformation = await documentSymbolProvider.provideDocumentSymbols(model, token);
+                if (token.isCancellationRequested) {
+                    return [];
+                }
+                if (Array.isArray(symbolInformation)) {
+                    entries.push(...symbolInformation);
+                }
+            } catch {
+                // happens if `provideDocumentSymbols` promise is rejected.
                 return [];
             }
-            if (Array.isArray(symbolInformation)) {
-                entries.push(...symbolInformation);
-            }
         }
 
         return entries;