Addressed feedback microsoft#1

- reset CustomScrollbar._asyncInitTimer to undefined when it fires
- made the deferred work in FocusManager idempotent

Author: ladipro

src/web/CustomScrollbar.ts

@@ -513,6 +513,7 @@ export class Scrollbar {
 
         // Defer remaining init work to avoid triggering sync layout
         this._asyncInitTimer = window.setTimeout(() => {
+            this._asyncInitTimer = undefined;
             this._tryLtrOverride();
             this.update();
         }, 0);

src/web/utils/FocusManager.ts

@@ -161,10 +161,15 @@ export class FocusManager extends FocusManagerBase {
             // back again.
             // Defer the work to avoid triggering sync layout.
             FocusManager._resetFocusTimer = setTimeout(() => {
+                FocusManager._resetFocusTimer = undefined;
                 const prevTabIndex = FocusManager._setTabIndex(document.body, 0);
+                const activeElement = document.activeElement;
                 document.body.focus();
                 document.body.blur();
                 FocusManager._setTabIndex(document.body, prevTabIndex);
+                if (activeElement instanceof HTMLElement) {
+                    activeElement.focus();
+                }
             }, 0);
         }
     }