Merge from vscode 81d7885dc2e9dc617e1522697a2966bc4025a45d (#5949)

* Merge from vscode 81d7885dc2e9dc617e1522697a2966bc4025a45d

* Fix vs unit tests and hygiene issue

* Fix strict null check issue

src/vs/base/browser/ui/scrollbar/abstractScrollbar.ts

@@ -108,6 +108,12 @@ export abstract class AbstractScrollbar extends Widget {
 				this._sliderMouseDown(e, () => { /*nothing to do*/ });
 			}
 		});
+
+		this.onclick(this.slider.domNode, e => {
+			if (e.leftButton) {
+				e.stopPropagation();
+			}
+		});
 	}
 
 	// ----------------- Update state