Fix synchronization of scrolling event flooding (microsoft#209022)

fixes microsoft#209020

Author: benibenj

src/vs/workbench/contrib/scrollLocking/browser/scrollLocking.ts

@@ -12,6 +12,7 @@ import { IKeybindingService } from 'vs/platform/keybinding/common/keybinding';
 import { SideBySideEditor } from 'vs/workbench/browser/parts/editor/sideBySideEditor';
 import { IWorkbenchContribution } from 'vs/workbench/common/contributions';
 import { IEditorPane, IEditorPaneScrollPosition, isEditorPaneWithScrolling } from 'vs/workbench/common/editor';
+import { ReentrancyBarrier } from 'vs/workbench/contrib/mergeEditor/browser/utils';
 import { IEditorService } from 'vs/workbench/services/editor/common/editorService';
 import { IStatusbarEntryAccessor, IStatusbarService, StatusbarAlignment } from 'vs/workbench/services/statusbar/browser/statusbar';
 
@@ -59,6 +60,9 @@ export class SyncScroll extends Disposable implements IWorkbenchContribution {
 		this.toggleStatusbarItem(this.isActive);
 	}
 
+	// makes sure that the onDidEditorPaneScroll is not called multiple times for the same event
+	private _reentrancyBarrier = new ReentrancyBarrier();
+
 	private trackVisiblePanes(): void {
 		this.paneDisposables.clear();
 		this.paneInitialScrollTop.clear();
@@ -70,7 +74,11 @@ export class SyncScroll extends Disposable implements IWorkbenchContribution {
 			}
 
 			this.paneInitialScrollTop.set(pane, pane.getScrollPosition());
-			this.paneDisposables.add(pane.onDidChangeScroll(() => this.onDidEditorPaneScroll(pane)));
+			this.paneDisposables.add(pane.onDidChangeScroll(() =>
+				this._reentrancyBarrier.runExclusively(() => {
+					this.onDidEditorPaneScroll(pane);
+				})
+			));
 		}
 	}